nparts parameter can't differ from the number of processors :(

Dear Sir or Madame,

The problem is as follows: I set nparts to number of processors and everything works. But when I set it to different number, I get the following mistake:
p0_15533: p4_error: interrupt SIGSEGV: 11

I thought that it's probably my mistake, but when I changed mtest.c file this way:
nparts = npes+3;
I got the same mistake.

Could you please tell me if this is an error in ParMetis library, or I should try to find my own fault here?

Sincerely,
Igor

P.S.: I'm using ParMetis-3.1 and mpich-1.2.7p1 for Linux.