diff --git a/plugins/mate/mate_grammar.lemon b/plugins/mate/mate_grammar.lemon
index f40243789f..302f0582a5 100644
--- a/plugins/mate/mate_grammar.lemon
+++ b/plugins/mate/mate_grammar.lemon
@@ -28,7 +28,9 @@
  * XXX - there's a Lemon bug where this grammar produces a parser that
  * fails assertions; to work around it, we disable assert() failures.
  */
+#ifndef NDEBUG
 #define NDEBUG
+#endif
 
 #include "mate.h"
 #include "mate_grammar.h"