I have a Fender Volume/Tone pedal where the swivel top is modded to be an expression pedal.  That means it has a normal volume pedal use but the “tone” swivel is an expression pedal.  It doesn’t work with the Riddle.  Absolutely nothing happens.  I plugged my Roland EV-5 expression pedal into the Riddle and it works as it should.  I tested both pedals on my Line 6 PODXT and on a cheap Zoom multi-fx.  The both work exactly as they should and appear to be equal. 

So, it appears as though the Riddle is finicky about expression pedals.  Can someone please explain to me why this is?  Are there certain expression pot values that won’t work with EHX productts?