Does dictation work with an aggregate device?

I'm trying to set up dictation to work both with a 3.5mm lapel mic connected directly to the imac and two xlr mics connected to an audio interface. When i set the dictation preferences to either one it works fine, but when i set it to an aggregate device that i created (or the automatic setting in these preferences), it doesnt work- the microphone on screen shows sound is being heard, but no words will be dictated. Is something wrong?

Thanks!