[Zope] - hmm

I'm getting a fair bit of messages like: Traceback (innermost last): File d:\local\src\Zope\lib\python\ZPublisher\Publish.py, line 860, in publish_module File d:\local\src\Zope\lib\python\ZPublisher\Publish.py, line 582, in publish (Info: /make_piechart) File d:\local\src\Zope\lib\python\Products\ExternalMethod\ExternalMethod.py, line 254, in __call__ (Object: Item) (Info: ((<Application instance at 0092CB70>, Response('')), {}, None)) TypeError: read-only character buffer, class when trying to look at images returned by external methods, in a frame which shows a 'broken picture' icon. What does it mean? --david