ConnectApi.NotFoundException: Resource not found

I am getting an error when using the ConnectApi to post a feed:

ConnectApi.NotFoundException: Resource not found. Class.ConnectApi.ChatterFeeds.getFeedElement: line 1502, column Class.ChatterPostParser: line 154, column 1 Class.RestComments.insertComment: line 66, column 1

I am trying this code:

List<User> mentionUsers = new List<User>();
ConnectApi.CommentInput feedCommentInput = 

ChatterPostParser.parseChatterCommentWithMentions(comment, mentionUsers);
ConnectApi.Comment commentRep = ConnectApi.ChatterFeeds.postCommentToFeedElement(
                                  Network.getNetworkId(),
                                  (Id)feedId,
                                  feedCommentInput,
                                  null);