diff --git a/Libraries/LibJS/Runtime/ReflectObject.cpp b/Libraries/LibJS/Runtime/ReflectObject.cpp
index 3fd2dddb04c..2a10fce38e4 100644
--- a/Libraries/LibJS/Runtime/ReflectObject.cpp
+++ b/Libraries/LibJS/Runtime/ReflectObject.cpp
@@ -261,9 +261,7 @@ Value ReflectObject::set_prototype_of(Interpreter& interpreter)
     Object* prototype = nullptr;
     if (!prototype_value.is_null())
         prototype = const_cast<Object*>(&prototype_value.as_object());
-    target->set_prototype(prototype);
-    // FIXME: Needs to return false for prototype chain cycles and non-extensible objects (don't have those yet).
-    return Value(true);
+    return Value(target->set_prototype(prototype));
 }
 
 }
diff --git a/Libraries/LibJS/Tests/Reflect.setPrototypeOf.js b/Libraries/LibJS/Tests/Reflect.setPrototypeOf.js
index bd7d7f4fd1b..81758ff3ec8 100644
--- a/Libraries/LibJS/Tests/Reflect.setPrototypeOf.js
+++ b/Libraries/LibJS/Tests/Reflect.setPrototypeOf.js
@@ -28,10 +28,14 @@ try {
     assert(Reflect.setPrototypeOf({}, Reflect.getPrototypeOf({})) === true);
 
     var o = {};
+    var p = { foo: "bar" };
     assert(o.foo === undefined);
-    assert(Reflect.setPrototypeOf(o, { foo: "bar" }) === true);
+    assert(Reflect.setPrototypeOf(o, p) === true);
     assert(o.foo === "bar");
 
+    Reflect.preventExtensions(o);
+    assert(Reflect.setPrototypeOf(o, {}) === false);
+    assert(Reflect.setPrototypeOf(o, p) === true);
 
     console.log("PASS");
 } catch (e) {