Argument Error while running Ligand_interface.py

Hello, 

I am facing trouble running the script ligand interface.py. I have prepared the pdb file and params file as instructed. I have also added the params file to the default directory and updated residue_types.txt file. The following error gets invoked. Please do help rectify the error.

Traceback (most recent call last):
  File "LigandInterface.py", line 139, in <module>
    sample_ligand_interface(pdb_filename, partners, ligand_params, jobs, job_output)
  File "LigandInterface.py", line 30, in sample_ligand_interface
    new_res_set = generate_nonstandard_residue_set(ligand_params)
  File "LigandInterface.py", line 102, in generate_nonstandard_residue_set
    res_set.read_files( params_list , atoms , mm_atoms , orbitals )
AttributeError: 'ResidueTypeSet' object has no attribute 'read_files'

I have attached the script below.