def METHOD_NAME(missing_pos_system):
    """
    Make sure the writing succeed when coordinates are missing and
    `nan_missing_pos` is `True`.
    """
    pdb_found = pdb.write_pdb_string(
        missing_pos_system,
        conect=False,
        omit_charges=False,
        nan_missing_pos=True,  # Argument of interest!
    )
    expected = '''