tutorial: "doesNotUnderstand" instance variable

I'm trying to go through the pipsqueak tutorial, but it throws a 
doesNotUnderstand error when I try to initialize the exitSides instance 
variable the first time.  The browser shows the exitSides variable (with 
nil value), so the VM knows its there.

This is my class declaration:
Object subclass: #BlankCell
   instanceVariableNames: 'activeSegments exitSides'
   classVariableNames: ''
   poolDictionaries: ''
   category: 'Laser-Game-Model'

This is my init function:
initializeExitSides
   self exitSides: Dictionary new.
   self exitSides at: #north put: #south.
   self exitSides at: #east put: #west.
   self exitSides at: #south put: #north.
   self exitSides at: #west put: #east.

And a screen shot of my error stack trace is here:
 http://www.isi.edu/~amarshal/transfers/squeak_error.png
(How do I copy it as text anyway?)


Anm