ConTeXt and forms - getting started?

[Michael W Daniels <daniels@ling.ohio-state.edu>]

I'm trying to create a pdf file that uses line and text fields. So I 
thought a good way to start would be to create a minimal file, to test that 
everything's installed properly (I'm using MiKTeX 2.2 on WinXP). Based on 
the widgets documentation file, the file below looks like something that 
ought to work.

Unfortunately, I just get a solid black box (exactly the right size, 
though) in the resulting pdf file. Have I missed a crucial command, or is 
something wrong with my setup?

Here's the test file I used:

\font\rm = tyxr at 12pt
\setupoutput[pdftex]
\setuppapersize[letter]
\setupbodyfont[12pt]
\setuppagenumber[state=stop]
\setupwhitespace[small]
\setupfield[ShortLine]
            [label,frame,horizontal]
            [offset=4pt,height=fit,framecolor=green,
             background=screen,backgroundscreen=.80]
            [height=18pt,width=80pt,align=middle,
             background=screen,backgroundscreen=.90,frame=off]
            [height=18pt,width=80pt,color=red,align=right,style=type,
             background=screen,backgroundscreen=.90,frame=off]
\definefield[Email][line][ShortLine][][pragma@wxs.nl]
\starttext
\rm\field[Email][your email]
\stoptext

Any help will be greatly appreciated!
-- 
Michael W. Daniels          | "The thunder was ominous-sounding, much
daniels@ling.ohio-state.edu |  like the sound of a thin sheet of metal
Department of Linguistics   |  being shaken backstage during the storm
218 Oxley Hall              |  scene in a play."