diff --git a/probe_nat.go b/probe_nat.go
index a36b841..44bff46 100644
--- a/probe_nat.go
+++ b/probe_nat.go
@@ -311,6 +311,8 @@ func DetectNAT(conn net.PacketConn, primarySTUN, secondarySTUN, network string,
 			res.Type = NATRestricted
 		case FilteringAddressPortDependent:
 			res.Type = NATPortRestricted
+		default:
+			res.Type = NATUnknown
 		}
 	} else {
 		res.Type = NATSymmetric