diff --git a/scard_darwin.go b/scard_darwin.go
index e17d190..cb421e3 100644
--- a/scard_darwin.go
+++ b/scard_darwin.go
@@ -124,7 +124,7 @@ func scardTransmit(card uintptr, proto Protocol, cmd []byte, rsp []byte) (uint32
 	case ProtocolT0, ProtocolT1:
 		sendpci.dwProtocol = C.uint32_t(proto)
 	default:
-		panic("unknown protocol")
+		return 0, Error(C.SCARD_F_INTERNAL_ERROR)
 	}
 	sendpci.cbPciLength = C.sizeof_SCARD_IO_REQUEST