CHARMM res number overflow

Dear CCL'ers
 I tried to perform energy minimization of a protein molecule immersed in a
 rectangular box of water molecules. The structure is after 1ns of MD.
 After SD started I got the message RESIDUE NUMBER OVERFLOW, even though I
 know it is impossible (I performed 1ns MD on exactly the same system and I
 minimized the structure at the begininnig of MD). The command coor orient
 helps a bit - I got this overflow message after a couple of minimization
 steps. There must be a problem with orienting water with respect to
 protein and/or image atoms. If you know how I could perform minimzation
 and what the problem actually is, please let me know.
 Thank you in advance
 Dr Marcin Krol
 Zaklad Biostatystyki Collegium Medicum UJ
 Kopernika 7E
 31-501 Krakow
 tel/fax (012) 422-77-64