1 files changed, 3 insertions, 2 deletions
diff --git a/Objects/stringlib/string_format.h b/Objects/stringlib/string_format.h
index ce7c909..ee6533e 100644
--- a/Objects/stringlib/string_format.h
+++ b/Objects/stringlib/string_format.h
@@ -375,8 +375,9 @@ FieldNameIterator_next(FieldNameIterator *self, int *is_attribute,
         *name_idx = get_integer(name);
         break;
     default:
-        /* interal error, can't get here */
-        assert(0);
+        /* Invalid character follows ']' */
+        PyErr_SetString(PyExc_ValueError, "Only '.' or '[' may "
+                        "follow ']' in format field specifier");
         return 0;
     }