diff --git a/lib/hx509/hx_locl.h b/lib/hx509/hx_locl.h
index dbf0d3f0f..301f09f6d 100644
--- a/lib/hx509/hx_locl.h
+++ b/lib/hx509/hx_locl.h
@@ -39,6 +39,7 @@
 
 #include <stdio.h>
 #include <stdlib.h>
+#include <ctype.h>
 #include <errno.h>
 #include <strings.h>
 #include <assert.h>