ERROR: MemoryContextAlloc: invalid request size

I'm getting errors like the following when I do queries/copies/dumps on
certain tables.
ERROR:  MemoryContextAlloc: invalid request size 4294967293
The number will sometimes differ. I'm running 7.1.3 on RedHat Linux 6.2.
Dual-proc with 2GB RAM. I upgraded to 7.1.3 a few weeks ago. This is the
first time this has happened. I can't think of any configuration changes I
made since installing it. I run the same import and export script every
night. Any clues would be appreciated.
Thanks,
Rich