why colabdesign alphafold limits on insertion code at chain

[rocke2020]

Dear sokrypton,
In colabdesign/af/alphafold/common/protein.py, there are limit on insertion code at chain as the code snippet below.
Could you teach why alphafold design has this limit of PDB data? thanks!!

  for res in chain:
    if res.id[2] != ' ':
      raise ValueError(
          f'PDB contains an insertion code at chain {chain.id} and residue '
          f'index {res.id[1]}. These are not supported.')