diff --git a/library/RSL_Emulation.ttcn b/library/RSL_Emulation.ttcn
index 3c035e908..c56c27833 100644
--- a/library/RSL_Emulation.ttcn
+++ b/library/RSL_Emulation.ttcn
@@ -318,7 +318,8 @@ private function f_streamId_by_trx(uint8_t trx_nr) return IpaStreamId {
 	case (2) { return IPAC_PROTO_RSL_TRX2; }
 	case (3) { return IPAC_PROTO_RSL_TRX3; }
 	}
-	self.stop;
+	setverdict(fail, "Unknown stream ID ", trx_nr);
+	mtc.stop;
 }