diff --git a/src/common/util.c b/src/common/util.c
index 45639e3426..243c5393b8 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -23,6 +23,9 @@
 #include <windows.h>
 #endif
 
+#ifdef HAVE_CTYPE_H
+#include <ctype.h>
+#endif
 #include <stdlib.h>
 #include <stdio.h>
 #include <string.h>