[Newbies] (drawing) doesNotUnderstand fillRectangle

I continue working on my stave-and-tabulature program(some older, but not very older version is in one of last mails). And I still cant really draw my object. Its rectangle (subclass of RectangleMorph) with submorph StringMorph - just a easy button. Here I have its drawOn:

drawOn: aCanvas |label| super drawOn: aCanvas. label:=self findA: StringMorph. label center: self center. label drawOn: self.

When I try to

but:=TempoButton new. but drawOn: World.

in Workspace and doIt, I have debugger with:

PasteUpMorph(Object)>>doesNotUnderstand: #fillRectangle:fillStyle:borderStyle

So I think it goes up in superclass chain, and finds out drawOn: of Morph, but for some reason it doesnt really understands it. Why?