diff --git a/libc/include/bits/fortify/stdlib.h b/libc/include/bits/fortify/stdlib.h
index cf4b7ea..8f3b02c 100644
--- a/libc/include/bits/fortify/stdlib.h
+++ b/libc/include/bits/fortify/stdlib.h
@@ -40,7 +40,8 @@
 #if defined(__clang__)
 char* realpath(const char* path, char* resolved)
         __clang_error_if(__bos(resolved) != __BIONIC_FORTIFY_UNKNOWN_SIZE &&
-                         __bos(resolved) < __PATH_MAX, __realpath_buf_too_small_str);
+                         __bos(resolved) < __PATH_MAX, __realpath_buf_too_small_str)
+        __clang_error_if(!path, "'realpath': NULL path is never correct; flipped arguments?");
 /* No need for a definition; the only issues we can catch are at compile-time. */
 
 #else /* defined(__clang__) */