pcall() in chdk lua

[Matt Kime <mattkime@...>]

I'm using lua on chdk which runs on a Canon Powershot a650IS, a point
and shoot camera. I'm trying to use pcall() in order to trap errors
I'm unable to eliminate in my scripts. more specifically, there are a
couple of chdk native functions (press("half_shoot"),
release("half_shoot"))which very occasionally fail. (1% to 0.1% of the
time) unfortunately calling these functions through pcall results in
the following error message -

attempt to yield across metamethod / c-call boundary

any advice on how to resolve this?

I realize that chdk lua specific issues may be at fault but any ideas
or suggestions on how to fix or work around this are greatly
appreciated.

--matt