diff --git a/apps/apps.c b/apps/apps.c
index b2a28c9b4..be7a80acb 100644
--- a/apps/apps.c
+++ b/apps/apps.c
@@ -114,6 +114,7 @@
 #include <string.h>
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <ctype.h>
 #include <openssl/err.h>
 #include <openssl/x509.h>
 #include <openssl/x509v3.h>