diff --git a/python/google/protobuf/pyext/unknown_fields.cc b/python/google/protobuf/pyext/unknown_fields.cc
index b9ca6ad3b0f31..6d919b361ad94 100644
--- a/python/google/protobuf/pyext/unknown_fields.cc
+++ b/python/google/protobuf/pyext/unknown_fields.cc
@@ -219,7 +219,7 @@ const UnknownField* GetUnknownField(PyUnknownFieldRef* self) {
                  "The parent message might be cleared.");
     return NULL;
   }
-  ssize_t total_size = fields->field_count();
+  Py_ssize_t total_size = fields->field_count();
   if (self->index >= total_size) {
     PyErr_Format(PyExc_ValueError,
                  "UnknownField does not exist. "