Custom Email post-function does not save multiple values selected in 'To issue fields' & 'cc issue fields'

Steps to Reproduce

1. Select more than one item in 'To issue fields' or 'cc issue field' of a custom email post-function (i.e. Reporter and Assignee) and click Update.
2. Reopen the edit page of the post function.

Expected Result

Parameters should be saved. 

Actual Result
'To issue fields' or 'cc issue fields' is empty.