Error "undefined method is_haml?" (custom form builder)

Hi all

I'm experimenting with HAML. Sadly I'm getting an error on a page that
doesn't have anything to do with HAML (it's a normal rhtml template)!

undefined method `is_haml?' for #<IncenseFormBuilder:0x31b0e88>

It's a custom form builder I have built, and I have no idea were this
is_haml? comes from...

Any idea? Thanks a lot.
Josh