Upload Error on Epic Link and Sprint on JIRA Agile

Whenever I try to upload test cases using the importer utility (latest), error below appear on the logs:

"customfield_10005": "Operation value must be a string"

I've traced the fields and they pertain to the Epic Link and Sprint on Agile JIRA. Attached is the sample screenshot of the error log. Other than that, it works :-) thanks zfjdevelopers for making this tool.