diff --git a/python/BUILD b/python/BUILD
index 92e9c496c5c3..ea1dac316044 100644
--- a/python/BUILD
+++ b/python/BUILD
@@ -203,5 +203,6 @@ py_extension(
         "//upb/util:compare",
         "//upb/util:def_to_proto",
         "//upb/util:required_fields",
+        "@utf8_range",
     ],
 )
diff --git a/python/convert.c b/python/convert.c
index 413c42f7359d..0c1174ba6b7a 100644
--- a/python/convert.c
+++ b/python/convert.c
@@ -35,6 +35,7 @@
 #include "upb/message/map.h"
 #include "upb/reflection/message.h"
 #include "upb/util/compare.h"
+#include "utf8_range.h"
 
 // Must be last.
 #include "upb/port/def.inc"
@@ -259,20 +260,27 @@ bool PyUpb_PyToUpb(PyObject* obj, const upb_FieldDef* f, upb_MessageValue* val,
     }
     case kUpb_CType_String: {
       Py_ssize_t size;
-      const char* ptr;
-      PyObject* unicode = NULL;
       if (PyBytes_Check(obj)) {
-        unicode = obj = PyUnicode_FromEncodedObject(obj, "utf-8", NULL);
-        if (!obj) return false;
+        // Use the object's bytes if they are valid UTF-8.
+        char* ptr;
+        if (PyBytes_AsStringAndSize(obj, &ptr, &size) < 0) return false;
+        if (utf8_range2((const unsigned char*)ptr, size) != 0) {
+          // Invalid UTF-8.  Try to convert the message to a Python Unicode
+          // object, even though we know this will fail, just to get the
+          // idiomatic Python error message.
+          obj = PyUnicode_FromEncodedObject(obj, "utf-8", NULL);
+          assert(!obj);
+          return false;
+        }
+        *val = PyUpb_MaybeCopyString(ptr, size, arena);
+        return true;
+      } else {
+        const char* ptr;
+        ptr = PyUnicode_AsUTF8AndSize(obj, &size);
+        if (PyErr_Occurred()) return false;
+        *val = PyUpb_MaybeCopyString(ptr, size, arena);
+        return true;
       }
-      ptr = PyUnicode_AsUTF8AndSize(obj, &size);
-      if (PyErr_Occurred()) {
-        Py_XDECREF(unicode);
-        return false;
-      }
-      *val = PyUpb_MaybeCopyString(ptr, size, arena);
-      Py_XDECREF(unicode);
-      return true;
     }
     case kUpb_CType_Message:
       PyErr_Format(PyExc_ValueError, "Message objects may not be assigned");
diff --git a/python/google/protobuf/internal/message_test.py b/python/google/protobuf/internal/message_test.py
index 0bac00da023d..b0f1ae784fa6 100755
--- a/python/google/protobuf/internal/message_test.py
+++ b/python/google/protobuf/internal/message_test.py
@@ -48,7 +48,6 @@
 
 warnings.simplefilter('error', DeprecationWarning)
 
-
 @_parameterized.named_parameters(('_proto2', unittest_pb2),
                                 ('_proto3', unittest_proto3_arena_pb2))
 @testing_refleaks.TestCase
diff --git a/python/map.c b/python/map.c
index a1d75de9a127..6bf12af43808 100644
--- a/python/map.c
+++ b/python/map.c
@@ -179,7 +179,7 @@ int PyUpb_MapContainer_AssignSubscript(PyObject* _self, PyObject* key,
   const upb_FieldDef* val_f = upb_MessageDef_Field(entry_m, 1);
   upb_Arena* arena = PyUpb_Arena_Get(self->arena);
   upb_MessageValue u_key, u_val;
-  if (!PyUpb_PyToUpb(key, key_f, &u_key, arena)) return -1;
+  if (!PyUpb_PyToUpb(key, key_f, &u_key, NULL)) return -1;
 
   if (val) {
     if (!PyUpb_PyToUpb(val, val_f, &u_val, arena)) return -1;
@@ -200,9 +200,8 @@ PyObject* PyUpb_MapContainer_Subscript(PyObject* _self, PyObject* key) {
   const upb_MessageDef* entry_m = upb_FieldDef_MessageSubDef(f);
   const upb_FieldDef* key_f = upb_MessageDef_Field(entry_m, 0);
   const upb_FieldDef* val_f = upb_MessageDef_Field(entry_m, 1);
-  upb_Arena* arena = PyUpb_Arena_Get(self->arena);
   upb_MessageValue u_key, u_val;
-  if (!PyUpb_PyToUpb(key, key_f, &u_key, arena)) return NULL;
+  if (!PyUpb_PyToUpb(key, key_f, &u_key, NULL)) return NULL;
   if (!map || !upb_Map_Get(map, u_key, &u_val)) {
     map = PyUpb_MapContainer_EnsureReified(_self);
     upb_Arena* arena = PyUpb_Arena_Get(self->arena);
@@ -256,9 +255,8 @@ static PyObject* PyUpb_MapContainer_Get(PyObject* _self, PyObject* args,
   const upb_MessageDef* entry_m = upb_FieldDef_MessageSubDef(f);
   const upb_FieldDef* key_f = upb_MessageDef_Field(entry_m, 0);
   const upb_FieldDef* val_f = upb_MessageDef_Field(entry_m, 1);
-  upb_Arena* arena = PyUpb_Arena_Get(self->arena);
   upb_MessageValue u_key, u_val;
-  if (!PyUpb_PyToUpb(key, key_f, &u_key, arena)) return NULL;
+  if (!PyUpb_PyToUpb(key, key_f, &u_key, NULL)) return NULL;
   if (map && upb_Map_Get(map, u_key, &u_val)) {
     return PyUpb_UpbToPy(u_val, val_f, self->arena);
   }