[Zope] zope runtime error after upgrade to 2.6.2

[Sean Kelley]

I get this error when calling a method that worked in Zope version 2.4:
RuntimeError
"external method could not be called because it is None"

Here is the code of the method:

for image in context.objectValues('Image'):
    if not image.hasProperty('original_id'):
        context.makeThumbnail(image.getId())

Any ideas?