diff --git a/src/index.ts b/src/index.ts
index 18e405d4..4321ec8c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -17,7 +17,7 @@ export const isRimrafOptions = (o: any): o is RimrafOptions =>
   !!o &&
   typeof o === 'object' &&
   typeOrUndef(o.preserveRoot, 'boolean') &&
-  typeOrUndef(o.preserveRoot, 'number') &&
+  typeOrUndef(o.tmp, 'string') &&
   typeOrUndef(o.maxRetries, 'number') &&
   typeOrUndef(o.retryDelay, 'number') &&
   typeOrUndef(o.backoff, 'number') &&
diff --git a/test/opt-arg.js b/test/opt-arg.js
index f1460ac7..9d2686fe 100644
--- a/test/opt-arg.js
+++ b/test/opt-arg.js
@@ -8,3 +8,73 @@ t.throws(() => optArg(true))
 t.throws(() => optArg(null))
 t.throws(() => optArg('hello'))
 t.throws(() => optArg({ maxRetries: 'banana' }))
+
+t.test('every kind of invalid option value', t => {
+  // skip them when it's undefined, and skip the case
+  // where they're all undefined, otherwise try every
+  // possible combination of the values here.
+  const badBool = [undefined, 1, null, 'x', {}]
+  const badNum = [undefined, true, false, null, 'x', '1', {}]
+  const badStr = [undefined, { toString: () => 'hi' }, /hi/, Symbol.for('hi')]
+  for (const preserveRoot of badBool) {
+    for (const tmp of badStr) {
+      for (const maxRetries of badNum) {
+        for (const retryDelay of badNum) {
+          for (const backoff of badNum) {
+            for (const maxBackoff of badNum) {
+              if (
+                preserveRoot === undefined &&
+                maxRetries === undefined &&
+                retryDelay === undefined &&
+                backoff === undefined &&
+                maxBackoff === undefined
+              ) {
+                continue
+              }
+              t.throws(() =>
+                optArg({
+                  preserveRoot,
+                  maxRetries,
+                  retryDelay,
+                  backoff,
+                  maxBackoff,
+                })
+              )
+            }
+          }
+        }
+      }
+    }
+  }
+  t.end()
+})
+
+t.test('test every allowed combination', t => {
+  const goodBool = [undefined, true, false]
+  // note that a few of these actually aren't *valid*,
+  // but it's verifying what the initial opt checker does.
+  const goodNum = [undefined, 1, Math.pow(2, 32), -1]
+  const goodStr = [undefined, 'hi']
+  for (const preserveRoot of goodBool) {
+    for (const tmp of goodStr) {
+      for (const maxRetries of goodNum) {
+        for (const retryDelay of goodNum) {
+          for (const backoff of goodNum) {
+            for (const maxBackoff of goodNum) {
+              t.ok(
+                optArg({
+                  preserveRoot,
+                  maxRetries,
+                  retryDelay,
+                  backoff,
+                  maxBackoff,
+                })
+              )
+            }
+          }
+        }
+      }
+    }
+  }
+  t.end()
+})