Of course there are no input files -- there are no :components. So maybe the WTF error is correct behavior in the case of 'asdf:fasl-op. But in case of 'asdf:monolithic-fasl-op, I think the correct behavior should simply be to omit the "empty" fasl, which is apparently what it had been doing previously. 

This can be replicated with for example the current master of gendl. It's allowed for a defsystem to have only :depends-on and no :components, isn't it?