diff --git a/src/jsc/bindings/JSMockFunction.cpp b/src/jsc/bindings/JSMockFunction.cpp
index 1c1c7fe70f6e..11d28401da26 100644
--- a/src/jsc/bindings/JSMockFunction.cpp
+++ b/src/jsc/bindings/JSMockFunction.cpp
@@ -838,7 +838,11 @@ JSC_DEFINE_HOST_FUNCTION(jsMockFunctionCall, (JSGlobalObject * lexicalGlobalObje
     }
 
     JSC::ArgList args = JSC::ArgList(callframe);
-    JSValue thisValue = callframe->thisValue();
+    // For `f()` calls where `f` resolves through a scope, JSC passes the resolved
+    // JSScope as the unconverted `this` argument and relies on the callee's ToThis to
+    // normalize it. Host functions never run ToThis, so normalize here to avoid leaking
+    // an engine-internal scope object through mockReturnThis / the contexts array.
+    JSValue thisValue = callframe->thisValue().toThis(globalObject, JSC::ECMAMode::strict());
     JSC::JSArray* argumentsArray = nullptr;
     {
         JSC::ObjectInitializationScope object(vm);
diff --git a/test/js/bun/test/mock-fn.test.js b/test/js/bun/test/mock-fn.test.js
index 7f6a244d9806..72434fed22d5 100644
--- a/test/js/bun/test/mock-fn.test.js
+++ b/test/js/bun/test/mock-fn.test.js
@@ -238,6 +238,18 @@ describe("mock()", () => {
     const obj = { fn };
     expect(obj.fn()).toBe(obj);
   });
+  test("mockReturnThis on a closure-captured mock does not leak a scope object", () => {
+    // When a mock is called as a bare identifier captured by a closure, JSC passes the
+    // enclosing environment record as the unconverted `this`. mockReturnThis must not hand
+    // that engine-internal scope object back to JavaScript (reading a TDZ binding from it
+    // surfaces the empty JSValue and crashes the interpreter on a later `typeof`).
+    const fn = jest.fn().mockReturnThis();
+    const capture = () => fn;
+    expect(capture()).toBe(fn);
+    const result = fn();
+    expect(result).toBeUndefined();
+    expect(typeof result === "function").toBe(false);
+  });
   if (isBun) {
     test("jest.fn(10) return value shorthand", () => {
       expect(jest.fn(10)()).toBe(10);