diff --git a/isisd/isis_misc.h b/isisd/isis_misc.h
index 91bd248..3b97589 100644
--- a/isisd/isis_misc.h
+++ b/isisd/isis_misc.h
@@ -35,7 +35,7 @@
  * return value of 0 indicates wrong input
  */
 int dotformat2buff (u_char *, u_char *);
-int sysid2buff (u_char *, u_char *);
+int sysid2buff (u_char *, const u_char *);
 
 /*
  * Printing functions