Resolve "Project output folder is not set (needed for FlexUnit)" error

I had flexunit working correctly across my AS3 unit-tests. Then I installed RubyMine and it screwed up my IntelliJ configuration somehow. Now, despite having an output-folder defined in my .iml, when I attempt to run FlexUnit I get the error:

Build configuration 'client' (module 'client'): Project output folder is not set (needed for FlexUnit)

I have tried uninstalling IntelliJ, installing the EAP, removing and re-adding output-folders, all in various combinations. Nothing works. Any ideas?