diff --git a/test/common.js b/test/common.js
index 0e59d192f8d9d1..351e3673d14b02 100644
--- a/test/common.js
+++ b/test/common.js
@@ -397,7 +397,10 @@ function runCallChecks(exitCode) {
 
 
 exports.mustCall = function(fn, expected) {
-  if (typeof expected !== 'number') expected = 1;
+  if (expected === undefined)
+    expected = 1;
+  else if (typeof expected !== 'number')
+    throw new TypeError(`Invalid expected value: ${expected}`);
 
   const context = {
     expected: expected,
diff --git a/test/parallel/test-common.js b/test/parallel/test-common.js
index 5ffe2c351503c1..59485eec4a327f 100644
--- a/test/parallel/test-common.js
+++ b/test/parallel/test-common.js
@@ -5,3 +5,11 @@ var assert = require('assert');
 common.globalCheck = false;
 global.gc = 42;  // Not a valid global unless --expose_gc is set.
 assert.deepStrictEqual(common.leakedGlobals(), ['gc']);
+
+assert.throws(function() {
+  common.mustCall(function() {}, 'foo');
+}, /^TypeError: Invalid expected value: foo$/);
+
+assert.throws(function() {
+  common.mustCall(function() {}, /foo/);
+}, /^TypeError: Invalid expected value: \/foo\/$/);