Index: uspace/lib/c/generic/io/chardev_srv.c
===================================================================
--- uspace/lib/c/generic/io/chardev_srv.c	(revision c4c60250a236ed30397cd89ce5cea5eaf59cfba5)
+++ uspace/lib/c/generic/io/chardev_srv.c	(revision ac307b25a02622848f9ee9d1bff85d6cd4c35ca2)
@@ -169,5 +169,8 @@
 			break;
 		default:
-			async_answer_0(callid, EINVAL);
+			if (srv->srvs->ops->def_handler != NULL)
+				srv->srvs->ops->def_handler(srv, callid, &call);
+			else
+				async_answer_0(callid, ENOTSUP);
 		}
 	}