FireGPG fails to load after opening certain messages.

Going directly to compose a new message works fine.  Also, composing a new message after viewing certain emails works fine.  But after opening certain emails (that I haven't been able to find a common characteristic for, except for the error below), FireGPG fails to load until GMail is reloaded.  The offending emails always report the following in the error console:

Error: boutonboxs[j].getAttribute("class") is null
Source File: chrome://firegpg/content/Webmails/Gmail/cgmail2.js
Line: 136

thanks,
tony