diff --git a/Python/bltinmodule.c b/Python/bltinmodule.c
index 291ef45..82fb9ad 100644
--- a/Python/bltinmodule.c
+++ b/Python/bltinmodule.c
@@ -1888,6 +1888,11 @@
             Py_DECREF(iter);
             return NULL;
         }
+        if (PyBytes_Check(result)) {
+            PyErr_SetString(PyExc_TypeError,
+                "sum() can't sum bytes [use b''.join(seq) instead]");
+            return NULL;
+        }
         if (PyByteArray_Check(result)) {
             PyErr_SetString(PyExc_TypeError,
                 "sum() can't sum bytes [use b''.join(seq) instead]");