How do I implement this workflow?

I have created a field on my project called "Fixed by" when I do a transition to "testable" I want to first set this new field to the current Assignee...

But it tells me that "type User is not a subtype of User"

maybe there is a better way to achieve what I want to do...

We want to have a "reopen" action send an issue in "testable" state back to whoever put it in "testable" and mark it as "reopened" or "open" (whichever)

So I was thinking I would save the user on Assignee in the "Fixed by" field on a "testable" state change....

Am I barking up the wrong tree?