this is a really tiny nit ... but I find this syntax doesn't make it clear that its checking an exception should be thrown.  In the cases where the exception isn't being returned for more checks, like this one, can you switch to the other syntax you use elsewhere: `an[ArgException] should be thrownBy {...}`
?

otherwise, this looks fine to me.  If you don't feel like fixing this now, that is fine, you can just merge it.