Hi
I try to model an heterodimer using this alignment

>P1;1DZB2
structureX:1DZB2:1:A:107:B::::
QVKLQQSGAELVKPGASVKLSCTASGFNIKDTYMHWVKQRPEQGLEWIGRIDPANGNTKYDPKFQGKATITADTSSNTAYLQLSSLTSEDTAVYYCAR-----WDWYFDVWGQGTTVTVSSG/
------------DIELTQSPSSMYTSLGERVTITCKASQDINSYLRWFQQKPGKSPKTLIYYATSLADGVPSRFSGSGSGQDYSLTISSLESDDTTTYYCLQHGESPYTFGGGTKLEIK---------*

>P1;atc
sequence:::::::::
-VKLQQSGGGLVQPGGSLKLSCAASGFDFSRYWMSWVRQAPGKGLEWIGEINPDSSTINYTPSLKDTFIISRDNAKNTLYLQMSKVRSEDTALYYCARQGYDGYHWYFDVWGQGTTVTVSSE/
SLDLGGGGGGGGDIQLTQSPSSLSASVGETVTLTCGASENIYGGLNWYQRKQGKSPQLLIYGATKLADGMSSRFSGGGSGRQYFLKITSLHPDDVATYYCQNVLSNPLTFGAGTKLELKRADAAPTVS*

each time it gives me this error message
define__595E> Number of selected atoms in sets 2 & 3 is not the same:      121      128

Can you tell me please what's wrong in my alignment file

script used:
# Homology modeling by the automodel class
#
# Demonstrates how to build multi-chain models, and symmetry restraints
#
from modeller import *
from modeller.automodel import * # Load the automodel class
log.verbose()
# Override the 'special_restraints' and 'user_after_single_model' methods:
class MyModel(automodel):
      def special_restraints(self, aln):
          # Constrain the A and B chains to be identical (but only restrain
            # the C-alpha atoms, to reduce the number of interatomic distances
            # that need to be calculated):
            s1 = selection(self.chains['A']).only_atom_types('CA')
            s2 = selection(self.chains['B']).only_atom_types('CA')
            self.restraints.symmetry.append(symmetry(s1, s2, 1.0))
      def user_after_single_model(self):
            # Report on symmetry violations greater than 1A after building
            # each model:
            self.restraints.symmetry.report(1.0)
env = environ()
# directories for input atom files
env.io.atom_files_directory = './'

a = MyModel(env,
            alnfile  = 'ali.pir',
            knowns   = '1DZB2',
            sequence = 'atc')
a.starting_model= 1
a.ending_model  = 1

a.make()