npruntime sample plugin crashes (Windows)

Hello,
I'm trying to learn how to write a NPAPI plugin that can be called
from javascript in the browser.  After a certain amount of head-
scratching, I was able to build the npruntime sample (from
http://mxr.mozilla.org/seamonkey/source/modules/plugin/samples/npruntime/).
The sample does create a window in the browser, but whenever I try
clicking on one of the buttons on the test page to call a method in
the plugin, I get a crash in my plugin.  The call stack indicates that
I have an access violation in mozutils.dll.  (In some cases, the call
stack indicates that this dll is calling into _RtlEnterCriticalSection
in ntdll.dll.)

I've tried some of the obvious things: switching between unicode and
ascii, changing the struct byte alignment, changing the calling
convention, trying to link to different libs in the gecko sdk.
Nothing I have thought of so far helps.

I built against the version 10 SDK, and I am running Firefox version
10.0.2.  The sample project is from 2007, so I am not sure if
something is out of date there.

Does anyone have any ideas on why I am crashing?

Wayne