diff --git a/modules/mol/base/src/impl/chain_impl.cc b/modules/mol/base/src/impl/chain_impl.cc
index ca2aeaad7c5df77bf7f2d2860311df43f9507ae4..adf5aeb8b88ea887c6af3252ca0fdea423eab516 100644
--- a/modules/mol/base/src/impl/chain_impl.cc
+++ b/modules/mol/base/src/impl/chain_impl.cc
@@ -409,7 +409,9 @@ int ChainImpl::GetIndexForResNumInSequence(const ResNum& number) const
   if (pos<0 || pos>=static_cast<int>(residue_list_.size())) {
     return -1;
   }
-  assert(residue_list_[pos]->GetNumber()==number);
+
+  if(residue_list_[pos]->GetNumber()!=number) return -1;
+
   return pos;
 }