CHARMM RESID error



Hi,
 I am new to using CHARMM.  I made a water box and tried to combine the
 waterbox pdb file with my protein pdb file using CHARMM and generate the
 final psf file.  But I got the following error:
 ** ERROR IN SEQRDR ** REPEATED RESID FOR   391 1391 100
 when it tried to read the sequence for my waterbox.  The RESID for my
 water box starts from 1 and with a segment ID WATR.  There is no water
 molecules in my protein pdb file.
 CHARMM read in my protein pdb file without a problem.  Only when it got to
 reading the water box.  I don't know why it says I have repeated RESID.
 Any suggestion will be helpful. Thanks
 Alice