I am writing a windowless NPAPI plugin and testing with Firefox 3.0.11 on stock Unbuntu 8.04. My plugin writes image data to an ximage data buffer and tries to write it back to a browser-supplied Drawable with an XPutImage() call. But firefox consistently crashes immediately with an assert when I run my plugin:

firefox: ../../src/xcb_io.c:228: _XSend: Assertion `!dpy->xcb->request_extra' failed.

If I comment out the XPutImage call then no assert, but of course then I don't see my image in the browser window.

A google search showed that this asssert is a common problem with Xlib programs, but I cannot find any info about what is causing it or how to avoid it. I am an X11 newbie but an experienced linux programmer. Any clue how I can go about solving this?