Cannot dock RNA-Protein complex

I am using RNA structures generated by the MC-Sym|MC-Fold algorithm and docking it with ER-alpha receptor protein(PDB ID:1SJ0).
I am getting the following error:
There was an inconsistency in your data

Error message

Second pdb file contains an unknown amino acid or nucleic acid base C

Make sure to use three letter code for bases and amino acid

Can anybody inform me as to what is wrong? Thank you.

As the error message says: Use a three letter code for your RNA, i.e. ADE, CYT, URI, …