peer_protocol/ut-holepunch/err-code.go      |  6 +++++-
 peer_protocol/ut-holepunch/err-code_test.go | 10 ++++++++++

diff --git a/peer_protocol/ut-holepunch/err-code.go b/peer_protocol/ut-holepunch/err-code.go
index 42b1db1adf3f31d8abbb707918d3de20d92e9c2a..7cc61db736fd474f2d3dd731e372079a3e6201ac 100644
--- a/peer_protocol/ut-holepunch/err-code.go
+++ b/peer_protocol/ut-holepunch/err-code.go
@@ -1,5 +1,9 @@
 package utHolepunch
 
+import (
+	"fmt"
+)
+
 type ErrCode uint32
 
 var _ error = ErrCode(0)
@@ -22,6 +26,6 @@ 		return "the target peer does not support the holepunch extension"
 	case NoSelf:
 		return "the target endpoint belongs to the relaying peer"
 	default:
-		panic(ec)
+		return fmt.Sprintf("error code %d", ec)
 	}
 }
diff --git a/peer_protocol/ut-holepunch/err-code_test.go b/peer_protocol/ut-holepunch/err-code_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..4553810920444f4f011d8bbdbc822ad1090888e4
--- /dev/null
+++ b/peer_protocol/ut-holepunch/err-code_test.go
@@ -0,0 +1,10 @@
+package utHolepunch
+
+import (
+	"math/rand"
+	"testing"
+)
+
+func TestUnknownErrCodeError(t *testing.T) {
+	ErrCode(rand.Uint32()).Error()
+}