As reported in <http://psf.upfronthosting.co.za/roundup/meta/issue148>, using the "make a copy" feature on an issue with more than one person on the nosy list results in a traceback. 
The bug is caused by the nosy list being concatenated using comma-space instead of just commas, when creating the target url for the "make a copy" link. The code that parses the url just splits using commas, so the additional space becomes part of the username, and thus the database lookup fails. For details, refer to the mentioned bug report.
The attached patch fixes the problem and contains a unittest - please review and merge!