diff --git a/iucvterm/src/getopt.c b/iucvterm/src/getopt.c
index 503f6dd8..b79b6a93 100644
--- a/iucvterm/src/getopt.c
+++ b/iucvterm/src/getopt.c
@@ -8,6 +8,7 @@
  * s390-tools is free software; you can redistribute it and/or modify
  * it under the terms of the MIT license. See LICENSE for details.
  */
+#include <ctype.h>
 #include <getopt.h>
 #include <stdio.h>
 #include <stdlib.h>