diff --git a/src/user/user_api.cc b/src/user/user_api.cc
index 6962493f..e21181dd 100644
--- a/src/user/user_api.cc
+++ b/src/user/user_api.cc
@@ -73,7 +73,7 @@ mjSpec* mj_copySpec(const mjSpec* s) {
   try {
     modelC = new mjCModel(*static_cast<mjCModel*>(s->element));
   } catch (mjCError& e) {
-    modelC->SetError(e);
+    static_cast<mjCModel*>(s->element)->SetError(e);
     return nullptr;
   }
   return &modelC->spec;
diff --git a/src/user/user_objects.cc b/src/user/user_objects.cc
index b3208662..827b5e48 100644
--- a/src/user/user_objects.cc
+++ b/src/user/user_objects.cc
@@ -916,7 +916,7 @@ mjCBody& mjCBody::operator+=(const mjCBody& other) {
 // attach frame to body
 mjCBody& mjCBody::operator+=(const mjCFrame& other) {
   // append a copy of the attached spec
-  if (other.model != model && !model->FindSpec(mjs_getString(other.model->spec.modelname))) {
+  if (other.model != model && !model->FindSpec(&other.model->spec.compiler)) {
     model->AppendSpec(mj_copySpec(&other.model->spec), &other.model->spec.compiler);
   }
 
@@ -2029,7 +2029,7 @@ mjCFrame& mjCFrame::operator=(const mjCFrame& other) {
 // attach body to frame
 mjCFrame& mjCFrame::operator+=(const mjCBody& other) {
   // append a copy of the attached spec
-  if (other.model != model && !model->FindSpec(mjs_getString(other.model->spec.modelname))) {
+  if (other.model != model && !model->FindSpec(&other.model->spec.compiler)) {
     model->AppendSpec(mj_copySpec(&other.model->spec), &other.model->spec.compiler);
   }
 
diff --git a/test/user/user_api_test.cc b/test/user/user_api_test.cc
index 993be5cd..4db9f25b 100644
--- a/test/user/user_api_test.cc
+++ b/test/user/user_api_test.cc
@@ -2552,31 +2552,46 @@ TEST_F(MujocoTest, DifferentOptionsInAttachedFrame) {
   // load specs and compile child
   mjSpec* parent = mj_parseXMLString(xml_parent, 0, nullptr, 0);
   EXPECT_THAT(parent, NotNull());
-  mjSpec* child = mj_parseXMLString(xml_child, 0, nullptr, 0);
-  EXPECT_THAT(child, NotNull());
-  mjModel* m_child = mj_compile(child, 0);
-  EXPECT_THAT(m_child, NotNull());
+  mjSpec* child1 = mj_parseXMLString(xml_child, 0, nullptr, 0);
+  EXPECT_THAT(child1, NotNull());
+  mjModel* m_child1 = mj_compile(child1, 0);
+  EXPECT_THAT(m_child1, NotNull());
+  mjSpec* child2 = mj_parseXMLString(xml_child, 0, nullptr, 0);
+  EXPECT_THAT(child2, NotNull());
+  mjModel* m_child2 = mj_compile(child1, 0);
+  EXPECT_THAT(m_child2, NotNull());
 
   // attach child frame to parent worldbody
   mjsBody* world = mjs_findBody(parent, "world");
   EXPECT_THAT(world, NotNull());
-  mjsFrame* child_frame = mjs_findFrame(child, "child");
-  EXPECT_THAT(child_frame, NotNull());
-  mjsElement* attached_frame =
-      mjs_attach(world->element, child_frame->element, "child-", "");
-  EXPECT_THAT(attached_frame, NotNull());
+  mjsFrame* child1_frame = mjs_findFrame(child1, "child");
+  EXPECT_THAT(child1_frame, NotNull());
+  mjsFrame* child2_frame = mjs_findFrame(child2, "child");
+  EXPECT_THAT(child2_frame, NotNull());
+  mjsElement* attached_frame1 =
+      mjs_attach(world->element, child1_frame->element, "child-", "-1");
+  EXPECT_THAT(attached_frame1, NotNull());
+  mjsElement* attached_frame2 =
+      mjs_attach(world->element, child2_frame->element, "child-", "-2");
+  EXPECT_THAT(attached_frame2, NotNull());
 
   // wrap the child frame in the parent frame and compile
   mjModel* m_attached = mj_compile(parent, 0);
   EXPECT_THAT(m_attached, NotNull());
-  EXPECT_NEAR(m_attached->site_quat[0], m_child->site_quat[0], 1e-6);
-  EXPECT_NEAR(m_attached->site_quat[1], m_child->site_quat[1], 1e-6);
-  EXPECT_NEAR(m_attached->site_quat[2], m_child->site_quat[2], 1e-6);
-  EXPECT_NEAR(m_attached->site_quat[3], m_child->site_quat[3], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[0], m_child1->site_quat[0], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[1], m_child1->site_quat[1], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[2], m_child1->site_quat[2], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[3], m_child1->site_quat[3], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[4], m_child2->site_quat[0], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[5], m_child2->site_quat[1], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[6], m_child2->site_quat[2], 1e-6);
+  EXPECT_NEAR(m_attached->site_quat[7], m_child2->site_quat[3], 1e-6);
 
   mj_deleteSpec(parent);
-  mj_deleteSpec(child);
-  mj_deleteModel(m_child);
+  mj_deleteSpec(child1);
+  mj_deleteModel(m_child1);
+  mj_deleteSpec(child2);
+  mj_deleteModel(m_child2);
   mj_deleteModel(m_attached);
 }