diff --git a/auth-sia.c b/auth-sia.c
index 63f55d0..af7182b 100644
--- a/auth-sia.c
+++ b/auth-sia.c
@@ -47,7 +47,7 @@
 extern char **saved_argv;
 
 int
-sys_auth_passwd(Authctxt *authctxt, char *pass)
+sys_auth_passwd(Authctxt *authctxt, const char *pass)
 {
 	int ret;
 	SIAENTITY *ent = NULL;