diff --git a/compiler/pre.h b/compiler/pre.h
index 19b19c7..55a748a 100644
--- a/compiler/pre.h
+++ b/compiler/pre.h
@@ -112,7 +112,7 @@ Str_to_c(Str s)
 {
 	if (s.len == 0 || s.s == nil)
 		return nil;
-	Assert(s.s[s.len - 1] == '\0');
+	Assert(s.s[s.len] == '\0');
 	return (char *)s.s;
 }