Windows service grabbing the screen in Python

Question

I have a Python script running as a Windows service that takes screenshots regularly. I had this working as a simple Python script, but now that I've turned it into a service it no longer functions. I get this output in Windows Event Viewer:

The instance's SvcRun() method failed 
Traceback (most recent call last):
File "C:\Python27\lib\site-packages\win32\lib\win32serviceutil.py", line 835, in SvcRun
    self.SvcDoRun()
File "D:\Malcolm\Dropbox\code\ambihue\ambiservice.py", line 58, in SvcDoRun
    r, g, b = ambihue.getAverageScreenColor()
File "D:\Malcolm\Dropbox\code\ambihue\ambihue.py", line 21, in getAverageScreenColor
    screen = ImageGrab.grab()
File "C:\Python27\lib\site-packages\PIL\ImageGrab.py", line 47, in grab
    size, data = grabber()
IOError: screen grab failed 
%2: %3

Here's how I get the screen:

screen = ImageGrab.grab()

Any suggestions on getting my service to successfully get the screen? Or do you have any alternate solutions for grabbing the screen - I'm using PIL right now.

My entire code is up here: https://github.com/crummy/ambihue