Evaluating gamma_incomplete() for large values



What I'm doing is somewhat silly but I ran into a limitation I can't 
seem to work around although I had assumed I could.

I can't seem to get numerical results from gamma_incomplete (and thus 
cdf_poisson()) for values above about 450000.

     bfloat(cdf_poisson(460000,460000));
     gamma_incomplete: series expansion failed for 
gamma_incomplete(4.60001b5, 4.6b5).

Are there setting that would allow me to do this or am I bumping into 
some fundamental numerical limit?

Regards,

Norman