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Invalid Identifier when using ConnectApi to add a comment to a case feed

I am trying to add a comment to a case feed using ConnectAPI.  
When I run this, I get the error

ConnectApi.ConnectApiException: Invalid identifier: 5003B000002o1FfQAI
Class.ConnectApi.ChatterFeeds.postCommentToFeedElement: line 674, column 1
Case salesforceCase;
createSFCase(salesforceCase);  //this works correctly and retrieving the Id gives me a value matching the system
ConnectApi.Comment comment = ConnectApi.ChatterFeeds.postCommentToFeedElement(null, salesforceCase.Id, 'comment text' );