diff --git a/crypto/x509/x509_cmp.c b/crypto/x509/x509_cmp.c
index 474f6de50d6bac721b76ede255a050722a58ca32..2b5aa09ad9c4a6fc334f786f6fa62a3250f43af4 100644
--- a/crypto/x509/x509_cmp.c
+++ b/crypto/x509/x509_cmp.c
@@ -57,6 +57,7 @@
  */
 
 #include <stdio.h>
+#include <ctype.h>
 #include "cryptlib.h"
 #include <openssl/asn1.h>
 #include <openssl/objects.h>