diff --git a/core/friend.go b/core/friend.go
index 78115c6..9620c9b 100644
--- a/core/friend.go
+++ b/core/friend.go
@@ -208,7 +208,7 @@ var errAddFriendFailed = errors.New("addFriend failed")
 
 func (fs *friends) addFriend(f *friendDoc) error {
 	_, newid, err := fs.mongoClient.Update(friends_collection_name, bson.M{
-		"_id": combineObjectID(f.From, f.To),
+		"_id": f.Id,
 	}, bson.M{
 		"$setOnInsert": f,
 	}, options.Update().SetUpsert(true))
diff --git a/core/invitation.go b/core/invitation.go
index 6d51dd6..b09af51 100644
--- a/core/invitation.go
+++ b/core/invitation.go
@@ -156,12 +156,14 @@ func (iv *invitation) AcceptInvitation(ctx wshandler.ApiCallContext) {
 
 	now := time.Now().UTC().Unix()
 	f1 := friendDoc{
+		Id:        combineObjectID(ivdoc.To, ivdoc.From),
 		From:      ivdoc.To,   // 수락한 나
 		To:        ivdoc.From, // 상대방
 		ToAlias:   ivdoc.FromAlias,
 		Timestamp: now,
 	}
 	f2 := friendDoc{
+		Id:        combineObjectID(ivdoc.From, ivdoc.To),
 		From:      ivdoc.From, // 상대방
 		To:        ivdoc.To,   // 나
 		ToAlias:   ivdoc.ToAlias,