diff --git a/kv/transaction/mvcc/write.go b/kv/transaction/mvcc/write.go
index 524f9511..831b3955 100644
--- a/kv/transaction/mvcc/write.go
+++ b/kv/transaction/mvcc/write.go
@@ -63,7 +63,7 @@ func WriteKindFromProto(op kvrpcpb.Op) WriteKind {
 		return WriteKindDelete
 	case kvrpcpb.Op_Rollback:
 		return WriteKindRollback
+	default:
+		panic("unsupported type")
 	}
-
-	return -1
 }