Copying issues with external links results in 500 Internal Server Error

When I try to copy a JIRA issue to another JIRA instance using JIRA to JIRA remote copy, I get a 500 internal server error (Java Nullpointer Exception) when the issue has a link pointing to an external site.

The issue is still copied, however the links are not copied over to the new issue. Also users do not know the issue did copy, so they try a few times creating duplicate issues in the other JIRA instance. As soon as I remove the external link from the source issue and try to copy again, no error occurs.

Also reported on Questions with the full error: https://answers.atlassian.com/questions/33155979/copying-issues-with-external-links-results-in-500-internal-server-error