CFI practical test - aircraft required

I am sure this subject has been brought up before (why? because the
answer is not immediately obvious in the FARs) but I'd appreciate
some input from the knowledge base.

I'm close to scheduling my CFI checkride (practical teat to be precise)
and although I've always assumed it had to be performed in a complex
airplane (and have been doing all the right seat flying in one), I can
not find a direct refrence in part 61 as to the requirement for such an
airplane. The most pertinent references I can find refer to training
and logging PIC in ".. the category and class of aircraft that is
appropriate to the flight instructor rating being sought".

This sort of reference appears several times throughout 61.185 but I
never see a direct reference to a "complex" airplane. So my question,
and that raised by my instructor today (gold seal BTW) is where did
this come from? He assured me he did his CFI checkride in a 152...

Any comments [and FAR references] are appreciated.