diff --git a/Python/ast.c b/Python/ast.c
index 424b601..aaf158f 100644
--- a/Python/ast.c
+++ b/Python/ast.c
@@ -3137,6 +3137,7 @@
     int imflag;
 #endif
 
+    assert(s != NULL);
     errno = 0;
     end = s + strlen(s) - 1;
 #ifndef WITHOUT_COMPLEX