diff --git a/src/hb-private.hh b/src/hb-private.hh
index d7c8fef8d..1b1e83f42 100644
--- a/src/hb-private.hh
+++ b/src/hb-private.hh
@@ -829,7 +829,7 @@ static inline unsigned char TOLOWER (unsigned char c)
  * light-weight) to be enabled, then HB_DEBUG can be defined to disable
  * the costlier checks. */
 #ifdef NDEBUG
-#define HB_NDEBUG
+#define HB_NDEBUG 1
 #endif