diff --git a/python/descriptor.c b/python/descriptor.c
index c60c8f50ee..815157c7a9 100644
--- a/python/descriptor.c
+++ b/python/descriptor.c
@@ -165,7 +165,18 @@ oom:
 static PyObject* PyUpb_DescriptorBase_CopyToProto(PyObject* _self,
                                                   PyUpb_ToProto_Func* func,
                                                   const upb_MiniTable* layout,
+                                                  const char* expected_type,
                                                   PyObject* py_proto) {
+  if (!PyUpb_CMessage_Verify(py_proto)) return NULL;
+  const upb_MessageDef* m = PyUpb_CMessage_GetMsgdef(py_proto);
+  const char* type = upb_MessageDef_FullName(m);
+  if (strcmp(type, expected_type) != 0) {
+    PyErr_Format(
+        PyExc_TypeError,
+        "CopyToProto: message is of incorrect type '%s' (expected '%s'", type,
+        expected_type);
+    return NULL;
+  }
   PyObject* serialized =
       PyUpb_DescriptorBase_GetSerializedProto(_self, func, layout);
   if (!serialized) return NULL;
@@ -319,7 +330,8 @@ static PyObject* PyUpb_Descriptor_CopyToProto(PyObject* _self,
                                               PyObject* py_proto) {
   return PyUpb_DescriptorBase_CopyToProto(
       _self, (PyUpb_ToProto_Func*)&upb_MessageDef_ToProto,
-      &google_protobuf_DescriptorProto_msginit, py_proto);
+      &google_protobuf_DescriptorProto_msginit,
+      "google.protobuf.DescriptorProto", py_proto);
 }
 
 static PyObject* PyUpb_Descriptor_EnumValueName(PyObject* _self,
@@ -728,7 +740,8 @@ static PyObject* PyUpb_EnumDescriptor_CopyToProto(PyObject* _self,
                                                   PyObject* py_proto) {
   return PyUpb_DescriptorBase_CopyToProto(
       _self, (PyUpb_ToProto_Func*)&upb_EnumDef_ToProto,
-      &google_protobuf_EnumDescriptorProto_msginit, py_proto);
+      &google_protobuf_EnumDescriptorProto_msginit,
+      "google.protobuf.EnumDescriptorProto", py_proto);
 }
 
 static PyGetSetDef PyUpb_EnumDescriptor_Getters[] = {
@@ -1248,7 +1261,8 @@ static PyObject* PyUpb_FileDescriptor_CopyToProto(PyObject* _self,
                                                   PyObject* py_proto) {
   return PyUpb_DescriptorBase_CopyToProto(
       _self, (PyUpb_ToProto_Func*)&upb_FileDef_ToProto,
-      &google_protobuf_FileDescriptorProto_msginit, py_proto);
+      &google_protobuf_FileDescriptorProto_msginit,
+      "google.protobuf.FileDescriptorProto", py_proto);
 }
 
 static PyGetSetDef PyUpb_FileDescriptor_Getters[] = {
@@ -1354,7 +1368,8 @@ static PyObject* PyUpb_MethodDescriptor_CopyToProto(PyObject* _self,
                                                     PyObject* py_proto) {
   return PyUpb_DescriptorBase_CopyToProto(
       _self, (PyUpb_ToProto_Func*)&upb_MethodDef_ToProto,
-      &google_protobuf_MethodDescriptorProto_msginit, py_proto);
+      &google_protobuf_MethodDescriptorProto_msginit,
+      "google.protobuf.MethodDescriptorProto", py_proto);
 }
 
 static PyGetSetDef PyUpb_MethodDescriptor_Getters[] = {
@@ -1559,7 +1574,8 @@ static PyObject* PyUpb_ServiceDescriptor_CopyToProto(PyObject* _self,
                                                      PyObject* py_proto) {
   return PyUpb_DescriptorBase_CopyToProto(
       _self, (PyUpb_ToProto_Func*)&upb_ServiceDef_ToProto,
-      &google_protobuf_ServiceDescriptorProto_msginit, py_proto);
+      &google_protobuf_ServiceDescriptorProto_msginit,
+      "google.protobuf.ServiceDescriptorProto", py_proto);
 }
 
 static PyObject* PyUpb_ServiceDescriptor_FindMethodByName(PyObject* _self,
diff --git a/python/descriptor_pool.c b/python/descriptor_pool.c
index 7428452760..06011d702e 100644
--- a/python/descriptor_pool.c
+++ b/python/descriptor_pool.c
@@ -249,7 +249,7 @@ done:
 
 static PyObject* PyUpb_DescriptorPool_DoAdd(PyObject* _self,
                                             PyObject* file_desc) {
-  if (!PyUpb_CMessage_Check(file_desc)) return NULL;
+  if (!PyUpb_CMessage_Verify(file_desc)) return NULL;
   const upb_MessageDef* m = PyUpb_CMessage_GetMsgdef(file_desc);
   const char* file_proto_name = "google.protobuf.FileDescriptorProto";
   if (strcmp(upb_MessageDef_FullName(m), file_proto_name) != 0) {
diff --git a/python/message.c b/python/message.c
index 9bede8d42b..11b3a0828e 100644
--- a/python/message.c
+++ b/python/message.c
@@ -232,7 +232,7 @@ bool PyUpb_CMessage_TryCheck(PyObject* self) {
   return Py_TYPE(type) == state->message_meta_type;
 }
 
-bool PyUpb_CMessage_Check(PyObject* self) {
+bool PyUpb_CMessage_Verify(PyObject* self) {
   if (!PyUpb_CMessage_TryCheck(self)) {
     PyErr_Format(PyExc_TypeError, "Expected a message object, but got %R.",
                  self);
@@ -1395,7 +1395,7 @@ PyObject* PyUpb_CMessage_SerializeInternal(PyObject* _self, PyObject* args,
                                            PyObject* kwargs,
                                            bool check_required) {
   PyUpb_CMessage* self = (void*)_self;
-  if (!PyUpb_CMessage_Check((PyObject*)self)) return NULL;
+  if (!PyUpb_CMessage_Verify((PyObject*)self)) return NULL;
   static const char* kwlist[] = {"deterministic", NULL};
   int deterministic = 0;
   if (!PyArg_ParseTupleAndKeywords(args, kwargs, "|p", (char**)(kwlist),
diff --git a/python/message.h b/python/message.h
index 73bac46910..46155ac8af 100644
--- a/python/message.h
+++ b/python/message.h
@@ -50,7 +50,7 @@ PyObject* PyUpb_CMessage_Get(upb_Message* u_msg, const upb_MessageDef* m,
 
 // Verifies that a Python object is a message.  Sets a TypeError exception and
 // returns false on failure.
-bool PyUpb_CMessage_Check(PyObject* self);
+bool PyUpb_CMessage_Verify(PyObject* self);
 
 // Gets the upb_Message* for this message object if the message is reified.
 // Otherwise returns NULL.
diff --git a/python/pb_unit_tests/descriptor_test_wrapper.py b/python/pb_unit_tests/descriptor_test_wrapper.py
index 92485e5b0b..758d86e974 100644
--- a/python/pb_unit_tests/descriptor_test_wrapper.py
+++ b/python/pb_unit_tests/descriptor_test_wrapper.py
@@ -26,10 +26,18 @@
 from google.protobuf.internal import descriptor_test
 import unittest
 
-descriptor_test.DescriptorCopyToProtoTest.testCopyToProto_TypeError.__unittest_expecting_failure__ = True
+# Our behavior here matches pure-Python, which does not allow
+# foo.enum_values_by_name.get([]).  We reject it because we return a true
+# dict (like pure Python), which does not allow hashing by a list.
 descriptor_test.GeneratedDescriptorTest.testDescriptor.__unittest_expecting_failure__ = True
+
+# These fail because they attempt to add fields with conflicting JSON names.
+# We don't want to support this going forward.
 descriptor_test.MakeDescriptorTest.testCamelcaseName.__unittest_expecting_failure__ = True
 descriptor_test.MakeDescriptorTest.testJsonName.__unittest_expecting_failure__ = True
+
+# We pass this test, but the error message is slightly different.
+# Our error message is better.
 descriptor_test.NewDescriptorTest.testImmutableCppDescriptor.__unittest_expecting_failure__ = True
 
 if __name__ == '__main__':
diff --git a/python/repeated.c b/python/repeated.c
index 3520b43a17..16407a508f 100644
--- a/python/repeated.c
+++ b/python/repeated.c
@@ -598,7 +598,7 @@ PyObject* PyUpb_RepeatedCompositeContainer_Add(PyObject* _self, PyObject* args,
 
 static PyObject* PyUpb_RepeatedCompositeContainer_Append(PyObject* _self,
                                                          PyObject* value) {
-  if (!PyUpb_CMessage_Check(value)) return NULL;
+  if (!PyUpb_CMessage_Verify(value)) return NULL;
   PyObject* py_msg = PyUpb_RepeatedCompositeContainer_AppendNew(_self);
   if (!py_msg) return NULL;
   PyObject* none = PyUpb_CMessage_MergeFrom(py_msg, value);