diff --git a/src/libregexp/lib9.std.h b/src/libregexp/lib9.std.h
index 0cefdde7..1d7aaa62 100644
--- a/src/libregexp/lib9.std.h
+++ b/src/libregexp/lib9.std.h
@@ -3,6 +3,7 @@
 #include <string.h>
 #include <stdlib.h>
 #include <unistd.h>
+#include <stddef.h>
 
 #define	exits(x)	exit(x && *x ? 1 : 0)