diff --git a/scripts/mktags.c b/scripts/mktags.c
index 3604260c4..9c23cf17e 100644
--- a/scripts/mktags.c
+++ b/scripts/mktags.c
@@ -3,6 +3,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <ctype.h>
 
 int main(int argc, char *argv[])
 {