diff --git a/src/silo/silo_ns.c b/src/silo/silo_ns.c
index 1c91eea9..56361503 100644
--- a/src/silo/silo_ns.c
+++ b/src/silo/silo_ns.c
@@ -49,6 +49,7 @@ reflect those  of the United  States Government or  Lawrence Livermore
 National  Security, LLC,  and shall  not  be used  for advertising  or
 product endorsement purposes.
 */
+#include <ctype.h>
 #include <stdarg.h>
 #include <stdlib.h>
 #include <errno.h>