diff --git a/Objects/unicodeobject.c b/Objects/unicodeobject.c
index 4756df7..3d8c895 100644
--- a/Objects/unicodeobject.c
+++ b/Objects/unicodeobject.c
@@ -9799,10 +9799,11 @@
             for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
             break; \
         } \
-        default: { \
+        case PyUnicode_4BYTE_KIND: { \
             Py_UCS4 * to_ = (Py_UCS4 *)((data)) + (start); \
             for (; i_ < (length); ++i_, ++to_) *to_ = (value); \
             break; \
+        default: assert(0); \
         } \
         } \
     } while (0)