diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index 9b59cb3574d18..179e5fb04d08e 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -21641,7 +21641,7 @@ namespace ts {
             if (isTupleType(restType)) {
                 const associatedNames = (<TupleType>(<TypeReference>restType).target).associatedNames;
                 const index = pos - paramCount;
-                return associatedNames ? associatedNames[index] : restParameter.escapedName + "_" + index as __String;
+                return associatedNames && associatedNames[index] || restParameter.escapedName + "_" + index as __String;
             }
             return restParameter.escapedName;
         }
diff --git a/tests/baselines/reference/getParameterNameAtPosition.errors.txt b/tests/baselines/reference/getParameterNameAtPosition.errors.txt
new file mode 100644
index 0000000000000..74a31fd3def13
--- /dev/null
+++ b/tests/baselines/reference/getParameterNameAtPosition.errors.txt
@@ -0,0 +1,20 @@
+tests/cases/compiler/getParameterNameAtPosition.ts(9,7): error TS2345: Argument of type 'Mock<[any]>' is not assignable to parameter of type 'Tester'.
+  Types of parameters 'args_1' and 'done' are incompatible.
+    Type '(...args: any[]) => any' is not assignable to type 'undefined'.
+
+
+==== tests/cases/compiler/getParameterNameAtPosition.ts (1 errors) ====
+    // Repro from #30171
+    
+    interface Mock<Y extends any[]> extends Function {
+        (...args: Y): any;
+    }
+    type Tester = (opts: any, done: (...args: any[]) => any) => any;
+    declare function cases(tester: Tester): void;
+    declare function fn<Y extends any[]>(implementation?: (...args: Y) => any): Mock<Y>;
+    cases(fn(opts => { }));
+          ~~~~~~~~~~~~~~~
+!!! error TS2345: Argument of type 'Mock<[any]>' is not assignable to parameter of type 'Tester'.
+!!! error TS2345:   Types of parameters 'args_1' and 'done' are incompatible.
+!!! error TS2345:     Type '(...args: any[]) => any' is not assignable to type 'undefined'.
+    
\ No newline at end of file
diff --git a/tests/baselines/reference/getParameterNameAtPosition.js b/tests/baselines/reference/getParameterNameAtPosition.js
new file mode 100644
index 0000000000000..fed4aebad6e0f
--- /dev/null
+++ b/tests/baselines/reference/getParameterNameAtPosition.js
@@ -0,0 +1,16 @@
+//// [getParameterNameAtPosition.ts]
+// Repro from #30171
+
+interface Mock<Y extends any[]> extends Function {
+    (...args: Y): any;
+}
+type Tester = (opts: any, done: (...args: any[]) => any) => any;
+declare function cases(tester: Tester): void;
+declare function fn<Y extends any[]>(implementation?: (...args: Y) => any): Mock<Y>;
+cases(fn(opts => { }));
+
+
+//// [getParameterNameAtPosition.js]
+"use strict";
+// Repro from #30171
+cases(fn(function (opts) { }));
diff --git a/tests/baselines/reference/getParameterNameAtPosition.symbols b/tests/baselines/reference/getParameterNameAtPosition.symbols
new file mode 100644
index 0000000000000..d8148a3733b31
--- /dev/null
+++ b/tests/baselines/reference/getParameterNameAtPosition.symbols
@@ -0,0 +1,37 @@
+=== tests/cases/compiler/getParameterNameAtPosition.ts ===
+// Repro from #30171
+
+interface Mock<Y extends any[]> extends Function {
+>Mock : Symbol(Mock, Decl(getParameterNameAtPosition.ts, 0, 0))
+>Y : Symbol(Y, Decl(getParameterNameAtPosition.ts, 2, 15))
+>Function : Symbol(Function, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+
+    (...args: Y): any;
+>args : Symbol(args, Decl(getParameterNameAtPosition.ts, 3, 5))
+>Y : Symbol(Y, Decl(getParameterNameAtPosition.ts, 2, 15))
+}
+type Tester = (opts: any, done: (...args: any[]) => any) => any;
+>Tester : Symbol(Tester, Decl(getParameterNameAtPosition.ts, 4, 1))
+>opts : Symbol(opts, Decl(getParameterNameAtPosition.ts, 5, 15))
+>done : Symbol(done, Decl(getParameterNameAtPosition.ts, 5, 25))
+>args : Symbol(args, Decl(getParameterNameAtPosition.ts, 5, 33))
+
+declare function cases(tester: Tester): void;
+>cases : Symbol(cases, Decl(getParameterNameAtPosition.ts, 5, 64))
+>tester : Symbol(tester, Decl(getParameterNameAtPosition.ts, 6, 23))
+>Tester : Symbol(Tester, Decl(getParameterNameAtPosition.ts, 4, 1))
+
+declare function fn<Y extends any[]>(implementation?: (...args: Y) => any): Mock<Y>;
+>fn : Symbol(fn, Decl(getParameterNameAtPosition.ts, 6, 45))
+>Y : Symbol(Y, Decl(getParameterNameAtPosition.ts, 7, 20))
+>implementation : Symbol(implementation, Decl(getParameterNameAtPosition.ts, 7, 37))
+>args : Symbol(args, Decl(getParameterNameAtPosition.ts, 7, 55))
+>Y : Symbol(Y, Decl(getParameterNameAtPosition.ts, 7, 20))
+>Mock : Symbol(Mock, Decl(getParameterNameAtPosition.ts, 0, 0))
+>Y : Symbol(Y, Decl(getParameterNameAtPosition.ts, 7, 20))
+
+cases(fn(opts => { }));
+>cases : Symbol(cases, Decl(getParameterNameAtPosition.ts, 5, 64))
+>fn : Symbol(fn, Decl(getParameterNameAtPosition.ts, 6, 45))
+>opts : Symbol(opts, Decl(getParameterNameAtPosition.ts, 8, 9))
+
diff --git a/tests/baselines/reference/getParameterNameAtPosition.types b/tests/baselines/reference/getParameterNameAtPosition.types
new file mode 100644
index 0000000000000..f722d7e5f9533
--- /dev/null
+++ b/tests/baselines/reference/getParameterNameAtPosition.types
@@ -0,0 +1,30 @@
+=== tests/cases/compiler/getParameterNameAtPosition.ts ===
+// Repro from #30171
+
+interface Mock<Y extends any[]> extends Function {
+    (...args: Y): any;
+>args : Y
+}
+type Tester = (opts: any, done: (...args: any[]) => any) => any;
+>Tester : Tester
+>opts : any
+>done : (...args: any[]) => any
+>args : any[]
+
+declare function cases(tester: Tester): void;
+>cases : (tester: Tester) => void
+>tester : Tester
+
+declare function fn<Y extends any[]>(implementation?: (...args: Y) => any): Mock<Y>;
+>fn : <Y extends any[]>(implementation?: ((...args: Y) => any) | undefined) => Mock<Y>
+>implementation : ((...args: Y) => any) | undefined
+>args : Y
+
+cases(fn(opts => { }));
+>cases(fn(opts => { })) : void
+>cases : (tester: Tester) => void
+>fn(opts => { }) : Mock<[any]>
+>fn : <Y extends any[]>(implementation?: ((...args: Y) => any) | undefined) => Mock<Y>
+>opts => { } : (opts: any) => void
+>opts : any
+
diff --git a/tests/cases/compiler/getParameterNameAtPosition.ts b/tests/cases/compiler/getParameterNameAtPosition.ts
new file mode 100644
index 0000000000000..e7a6dc4082766
--- /dev/null
+++ b/tests/cases/compiler/getParameterNameAtPosition.ts
@@ -0,0 +1,11 @@
+// @strict: true
+
+// Repro from #30171
+
+interface Mock<Y extends any[]> extends Function {
+    (...args: Y): any;
+}
+type Tester = (opts: any, done: (...args: any[]) => any) => any;
+declare function cases(tester: Tester): void;
+declare function fn<Y extends any[]>(implementation?: (...args: Y) => any): Mock<Y>;
+cases(fn(opts => { }));