diff --git a/Python/ast.c b/Python/ast.c
index 389fedd..9e8df86 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -3184,6 +3184,7 @@
         int imflag;
 #endif
 
+        assert(s != NULL);
         errno = 0;
         end = s + strlen(s) - 1;
 #ifndef WITHOUT_COMPLEX