diff --git a/lib/sinon/assert.js b/lib/sinon/assert.js
index ffd34e550..5af71bbb6 100644
--- a/lib/sinon/assert.js
+++ b/lib/sinon/assert.js
@@ -159,10 +159,16 @@ function createAssertObject() {
         callCount: function assertCallCount(method, count) {
             verifyIsStub(method);
 
-            if (method.callCount !== count) {
-                var msg = `expected %n to be called ${timesInWords(
-                    count
-                )} but was called %c%C`;
+            var msg;
+            if (typeof count !== "number") {
+                msg =
+                    `expected ${format(count)} to be a number ` +
+                    `but was of type ${typeof count}`;
+                failAssertion(this, msg);
+            } else if (method.callCount !== count) {
+                msg =
+                    `expected %n to be called ${timesInWords(count)} ` +
+                    `but was called %c%C`;
                 failAssertion(this, method.printf(msg));
             } else {
                 assert.pass("callCount");
diff --git a/lib/sinon/proxy-call.js b/lib/sinon/proxy-call.js
index ef81e8806..e76e93dcd 100644
--- a/lib/sinon/proxy-call.js
+++ b/lib/sinon/proxy-call.js
@@ -220,7 +220,8 @@ var callProto = {
             }
         }
         if (this.stack) {
-            // Omit the error message and the two top stack frames in sinon itself:
+            // If we have a stack, add the first frame that's in end-user code
+            // Skip the first two frames because they will refer to Sinon code
             callStr += (this.stack.split("\n")[3] || "unknown").replace(
                 /^\s*(?:at\s+|@)?/,
                 " at "
diff --git a/test/assert-test.js b/test/assert-test.js
index 95e49d8ad..a0db5c896 100644
--- a/test/assert-test.js
+++ b/test/assert-test.js
@@ -1438,7 +1438,9 @@ describe("assert", function () {
                         [].slice.call(arguments, 1)
                     );
                 } catch (e) {
-                    return e.message;
+                    // We sometimes append stack frames to the message and they
+                    // make assertions messy, so strip those off here
+                    return e.message.replace(/( at.*\(.*\)$)+/gm, "");
                 }
             };
         });
@@ -1454,10 +1456,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message("notCalled", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("notCalled", this.obj.doSomething),
                 "expected doSomething to not have been called but was called once\n    doSomething()"
             );
         });
@@ -1469,10 +1468,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message("notCalled", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("notCalled", this.obj.doSomething),
                 "expected doSomething to not have been called " +
                     "but was called 4 times\n    doSomething()\n    " +
                     "doSomething()\n    doSomething()\n    doSomething()"
@@ -1486,10 +1482,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message("notCalled", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("notCalled", this.obj.doSomething),
                 "expected doSomething to not have been called " +
                     "but was called 4 times\n    doSomething()\n    " +
                     "doSomething(3)\n    doSomething(42, 1)\n    doSomething()"
@@ -1573,33 +1566,33 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message("callCount", this.obj.doSomething, 3).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("callCount", this.obj.doSomething, 3),
                 "expected doSomething to be called thrice but was called once\n    doSomething()"
             );
         });
 
+        it("assert.callCount exception message with non-numeric argument", function () {
+            this.obj.doSomething();
+
+            assert.equals(
+                this.message("callCount", this.obj.doSomething, "3"),
+                "expected '3' to be a number but was of type string"
+            );
+        });
+
         it("assert.calledOnce exception message", function () {
             this.obj.doSomething();
             this.obj.doSomething();
 
             assert.equals(
-                this.message("calledOnce", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("calledOnce", this.obj.doSomething),
                 "expected doSomething to be called once but was called twice\n    doSomething()\n    doSomething()"
             );
 
             this.obj.doSomething();
 
             assert.equals(
-                this.message("calledOnce", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("calledOnce", this.obj.doSomething),
                 "expected doSomething to be called once but was called " +
                     "thrice\n    doSomething()\n    doSomething()\n    doSomething()"
             );
@@ -1609,10 +1602,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message("calledTwice", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("calledTwice", this.obj.doSomething),
                 "expected doSomething to be called twice but was called once\n    doSomething()"
             );
         });
@@ -1624,10 +1614,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message("calledThrice", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("calledThrice", this.obj.doSomething),
                 "expected doSomething to be called thrice but was called 4 times\n" +
                     "    doSomething()\n    doSomething()\n    doSomething()\n    doSomething()"
             );
@@ -1715,13 +1702,7 @@ describe("assert", function () {
             this.obj.doSomething(4, 3, "hey");
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    1,
-                    3,
-                    "hey"
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, 1, 3, "hey"),
                 `expected doSomething to be called with arguments \n${color.red(
                     "4"
                 )} ${color.green("1")} \n3\n${inspect('"hey"')}`
@@ -1733,13 +1714,7 @@ describe("assert", function () {
             this.obj.doSomething(1, 3, "not");
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    1,
-                    3,
-                    "hey"
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, 1, 3, "hey"),
                 `${
                     "expected doSomething to be called with arguments \n" +
                     "Call 1:\n"
@@ -1825,10 +1800,7 @@ describe("assert", function () {
             this.obj.doSomething(4);
 
             assert.equals(
-                this.message("calledWith", this.obj.doSomething, 1, 3).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("calledWith", this.obj.doSomething, 1, 3),
                 `expected doSomething to be called with arguments \n${color.red(
                     "4"
                 )} ${color.green("1")} \n${color.green("3")}`
@@ -1839,10 +1811,7 @@ describe("assert", function () {
             this.obj.doSomething(4, 3);
 
             assert.equals(
-                this.message("calledWith", this.obj.doSomething, 1).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("calledWith", this.obj.doSomething, 1),
                 `expected doSomething to be called with arguments \n${color.red(
                     "4"
                 )} ${color.green("1")} \n${color.red("3")}`
@@ -1858,7 +1827,7 @@ describe("assert", function () {
                     this.obj.doSomething,
                     match.any,
                     false
-                ).replace(/ at.*/g, ""),
+                ),
                 `${
                     "expected doSomething to be called with arguments \n" +
                     "true any\n"
@@ -1870,11 +1839,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    match.defined
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, match.defined),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "defined"
                 )}`
@@ -1885,11 +1850,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    match.truthy
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, match.truthy),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "truthy"
                 )}`
@@ -1900,11 +1861,7 @@ describe("assert", function () {
             this.obj.doSomething(true);
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    match.falsy
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, match.falsy),
                 `expected doSomething to be called with arguments \n${color.green(
                     "true"
                 )} ${color.red("falsy")}`
@@ -1915,11 +1872,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    match.same(1)
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, match.same(1)),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "same(1)"
                 )}`
@@ -1931,11 +1884,7 @@ describe("assert", function () {
             var matcher = match.typeOf("string");
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    matcher
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, matcher),
                 `expected doSomething to be called with arguments \n ${color.red(
                     'typeOf("string")'
                 )}`
@@ -1949,11 +1898,7 @@ describe("assert", function () {
             });
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    matcher
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, matcher),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "instanceOf(CustomType)"
                 )}`
@@ -1965,11 +1910,7 @@ describe("assert", function () {
             var matcher = match({ some: "value", and: 123 });
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    matcher
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, matcher),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "match(some: value, and: 123)"
                 )}`
@@ -1980,11 +1921,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    match(true)
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, match(true)),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "match(true)"
                 )}`
@@ -1995,11 +1932,7 @@ describe("assert", function () {
             this.obj.doSomething();
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    match(123)
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, match(123)),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "match(123)"
                 )}`
@@ -2011,11 +1944,7 @@ describe("assert", function () {
             var matcher = match("Sinon");
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    matcher
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, matcher),
                 `expected doSomething to be called with arguments \n ${color.red(
                     'match("Sinon")'
                 )}`
@@ -2030,7 +1959,7 @@ describe("assert", function () {
                     "calledWith",
                     this.obj.doSomething,
                     match(/[a-z]+/)
-                ).replace(/ at.*/g, ""),
+                ),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "match(/[a-z]+/)"
                 )}`
@@ -2046,11 +1975,7 @@ describe("assert", function () {
             });
 
             assert.equals(
-                this.message(
-                    "calledWith",
-                    this.obj.doSomething,
-                    matcher
-                ).replace(/ at.*/g, ""),
+                this.message("calledWith", this.obj.doSomething, matcher),
                 `expected doSomething to be called with arguments \n ${color.red(
                     "match(custom)"
                 )}`
@@ -2067,7 +1992,7 @@ describe("assert", function () {
                     4,
                     3,
                     "hey"
-                ).replace(/ at.*/g, ""),
+                ),
                 `expected doSomething to be called with match \n${color.red(
                     "1"
                 )} ${color.green("4")} \n3\n${inspect('"hey"')}`
@@ -2084,7 +2009,7 @@ describe("assert", function () {
                     this.obj.doSomething,
                     1,
                     "hey"
-                ).replace(/ at.*/g, ""),
+                ),
                 `${
                     "expected doSomething to always be called with arguments \n" +
                     "Call 1:\n" +
@@ -2107,7 +2032,7 @@ describe("assert", function () {
                     this.obj.doSomething,
                     1,
                     "hey"
-                ).replace(/ at.*/g, ""),
+                ),
                 `${
                     "expected doSomething to always be called with match \n" +
                     "Call 1:\n" +
@@ -2124,12 +2049,7 @@ describe("assert", function () {
             this.obj.doSomething(1, 3, "hey");
 
             assert.equals(
-                this.message(
-                    "calledWithExactly",
-                    this.obj.doSomething,
-                    1,
-                    3
-                ).replace(/ at.*/g, ""),
+                this.message("calledWithExactly", this.obj.doSomething, 1, 3),
                 `expected doSomething to be called with exact arguments \n1\n3\n${color.red(
                     inspect('"hey"')
                 )}`
@@ -2144,7 +2064,7 @@ describe("assert", function () {
                     1,
                     3,
                     "bob"
-                ).replace(/ at.*/g, ""),
+                ),
                 "expected doSomething to be called once and with exact arguments "
             );
 
@@ -2156,7 +2076,7 @@ describe("assert", function () {
                     1,
                     3,
                     "bob"
-                ).replace(/ at.*/g, ""),
+                ),
                 `expected doSomething to be called once and with exact arguments \n${color.red(
                     "4"
                 )} ${color.green("1")} \n3\n${inspect('"bob"')}`
@@ -2164,10 +2084,7 @@ describe("assert", function () {
 
             this.obj.doSomething();
             assert.equals(
-                this.message(
-                    "calledOnceWithExactly",
-                    this.obj.doSomething
-                ).replace(/ at.*/g, ""),
+                this.message("calledOnceWithExactly", this.obj.doSomething),
                 `${
                     "expected doSomething to be called once and with exact arguments \n" +
                     "Call 1:\n"
@@ -2198,7 +2115,7 @@ describe("assert", function () {
                     this.obj.doSomething,
                     1,
                     3
-                ).replace(/ at.*/g, ""),
+                ),
                 `${
                     "expected doSomething to always be called with exact arguments \n" +
                     "Call 1:\n" +
@@ -2215,12 +2132,7 @@ describe("assert", function () {
             this.obj.doSomething(1, 2, 3);
 
             assert.equals(
-                this.message(
-                    "neverCalledWith",
-                    this.obj.doSomething,
-                    1,
-                    2
-                ).replace(/ at.*/g, ""),
+                this.message("neverCalledWith", this.obj.doSomething, 1, 2),
                 "expected doSomething to never be called with arguments 1, 2\n    doSomething(1, 2, 3)"
             );
         });
@@ -2234,7 +2146,7 @@ describe("assert", function () {
                     this.obj.doSomething,
                     1,
                     2
-                ).replace(/ at.*/g, ""),
+                ),
                 "expected doSomething to never be called with match 1, 2\n    doSomething(1, 2, 3)"
             );
         });
@@ -2244,10 +2156,7 @@ describe("assert", function () {
             this.obj.doSomething(1, 3);
 
             assert.equals(
-                this.message("threw", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("threw", this.obj.doSomething),
                 "doSomething did not throw exception\n    doSomething(1, 3, 'hey')\n    doSomething(1, 3)"
             );
         });
@@ -2257,10 +2166,7 @@ describe("assert", function () {
             this.obj.doSomething(1, 3);
 
             assert.equals(
-                this.message("alwaysThrew", this.obj.doSomething).replace(
-                    / at.*/g,
-                    ""
-                ),
+                this.message("alwaysThrew", this.obj.doSomething),
                 "doSomething did not always throw exception\n    doSomething(1, 3, 'hey')\n    doSomething(1, 3)"
             );
         });
diff --git a/test/proxy-call-test.js b/test/proxy-call-test.js
index 5b8dd5438..a9871a11a 100644
--- a/test/proxy-call-test.js
+++ b/test/proxy-call-test.js
@@ -1176,7 +1176,7 @@ describe("sinonSpy.call", function () {
 
         // https://github.com/sinonjs/sinon/issues/1066
         /* eslint-disable consistent-return */
-        it("does not throw when the call stack is empty", function (done) {
+        it("does not throw when the call stack is empty", async function () {
             if (typeof Promise === "undefined") {
                 this.skip();
             }
@@ -1184,21 +1184,36 @@ describe("sinonSpy.call", function () {
             var stub1 = sinonStub().resolves(1);
             var stub2 = sinonStub().returns(1);
 
-            function run() {
-                return stub1().then(stub2);
-            }
+            await stub2(await stub1());
 
-            run()
-                .then(function () {
-                    assert.equals(
-                        stub2.getCall(0).toString().replace(/ at.*/g, ""),
-                        "stub(1) => 1"
-                    );
-                    done();
-                })
-                .catch(done);
+            assert.equals(
+                stub2.getCall(0).toString().replace(/ at.*/g, ""),
+                "stub(1) => 1"
+            );
         });
         /* eslint-enable consistent-return */
+
+        it("includes first stack entry from end-user code", function () {
+            /* We find the first stack frame that points to end-user code and
+             * add it to the error message. We do this by chopping off a
+             * constant number of stack frames, so if this test fails, you
+             * probably need to chop off a different number of frames
+             */
+            if (typeof __filename === "undefined") {
+                this.skip();
+            }
+
+            var object = { doIt: sinonSpy() };
+            object.doIt();
+
+            const [name, stackFrame] = object.doIt
+                .getCall(0)
+                .toString()
+                .split(" at ");
+
+            assert.equals(name, "doIt()");
+            assert.contains(stackFrame, __filename);
+        });
     });
 
     describe("constructor", function () {