diff --git a/lib/vty.c b/lib/vty.c
index 42b7e92..d21f38c 100644
--- a/lib/vty.c
+++ b/lib/vty.c
@@ -2422,7 +2422,7 @@
 
 /* Set time out value. */
 int
-exec_timeout (struct vty *vty, char *min_str, char *sec_str)
+exec_timeout (struct vty *vty, const char *min_str, const char *sec_str)
 {
   unsigned long timeout = 0;