diff --git a/lib/libc/ansi/malloc.c b/lib/libc/ansi/malloc.c
index 2c53dcbaa..d6990fef8 100644
--- a/lib/libc/ansi/malloc.c
+++ b/lib/libc/ansi/malloc.c
@@ -2,7 +2,7 @@
 
 /* replace undef by define */
 #define	 DEBUG		/* check assertions */
-#define	 SLOWDEBUG	/* some extra test loops (requires DEBUG) */
+#undef	 SLOWDEBUG	/* some extra test loops (requires DEBUG) */
 
 #ifndef DEBUG
 #define NDEBUG