error: failed while ordering equations

Dear Listers

Not come across this error message before, not in manual. The analysis
is of a single trait, approx 700 records, a few fixed effects, one
random effect for a largish pedigree (maybe 10,000 individuals).

Any ideas what might be cause?

--
  I White
  University of Edinburgh
  Ashworth Laboratories, West Mains Road
  Edinburgh EH9 3JT  (Tel 0131 650 5490)
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.