diff --git a/Python/pystrtod.c b/Python/pystrtod.c
index 9bf9363..141a47a 100644
--- a/Python/pystrtod.c
+++ b/Python/pystrtod.c
@@ -391,6 +391,8 @@
     char *dup, *end;
     PyObject *result;
 
+    assert(s[orig_len] == '\0');
+
     if (strchr(s, '_') == NULL) {
         return innerfunc(s, orig_len, arg);
     }