diff --git a/src/bun.js/bindings/BunPlugin.cpp b/src/bun.js/bindings/BunPlugin.cpp
index f5abe87b3cc..e6f847749ef 100644
--- a/src/bun.js/bindings/BunPlugin.cpp
+++ b/src/bun.js/bindings/BunPlugin.cpp
@@ -526,6 +526,12 @@ extern "C" JSC_DEFINE_HOST_FUNCTION(JSMock__jsModuleMock, (JSC::JSGlobalObject *
         return {};
     }
 
+    JSC::JSValue callbackValue = callframe->argument(1);
+    if (!callbackValue.isCell() || !callbackValue.isCallable()) {
+        scope.throwException(lexicalGlobalObject, JSC::createTypeError(lexicalGlobalObject, "mock(module, fn) requires a function"_s));
+        return {};
+    }
+
     auto resolveSpecifier = [&]() -> void {
         JSC::SourceOrigin sourceOrigin = callframe->callerSourceOrigin(vm);
         if (sourceOrigin.isNull())
@@ -581,12 +587,6 @@ extern "C" JSC_DEFINE_HOST_FUNCTION(JSMock__jsModuleMock, (JSC::JSGlobalObject *
     resolveSpecifier();
     RETURN_IF_EXCEPTION(scope, {});
 
-    JSC::JSValue callbackValue = callframe->argument(1);
-    if (!callbackValue.isCell() || !callbackValue.isCallable()) {
-        scope.throwException(lexicalGlobalObject, JSC::createTypeError(lexicalGlobalObject, "mock(module, fn) requires a function"_s));
-        return {};
-    }
-
     JSC::JSObject* callback = callbackValue.getObject();
 
     JSModuleMock* mock = JSModuleMock::create(vm, globalObject->mockModule.mockModuleStructure.getInitializedOnMainThread(globalObject), callback);
diff --git a/test/js/bun/test/mock/mock-module-non-string.test.ts b/test/js/bun/test/mock/mock-module-non-string.test.ts
index f7137f1ea64..cbbb37aa03c 100644
--- a/test/js/bun/test/mock/mock-module-non-string.test.ts
+++ b/test/js/bun/test/mock/mock-module-non-string.test.ts
@@ -16,3 +16,20 @@ test("mock.module still works with valid string argument", async () => {
   const m = await import("mock-module-non-string-test-fixture");
   expect(m.default).toBe(42);
 });
+
+test("mock.module throws TypeError without resolving when callback is missing", () => {
+  // Running the resolver on an arbitrary user-provided string can enter the
+  // package-manager auto-install code path and crash. When the caller didn't
+  // pass a callable second argument, we must fail fast before touching the
+  // resolver.
+  const specifiers = [
+    "function f3() {}",
+    "function foo(a, b) { return a + b; }",
+    "() => 1",
+    "some bogus package name {with braces}",
+  ];
+  for (const specifier of specifiers) {
+    // @ts-expect-error missing callback on purpose
+    expect(() => mock.module(specifier)).toThrow("mock(module, fn) requires a function");
+  }
+});