[Biopython] Polypeptide.is_aa of GDP = True?

[Ahmad Khalifa]

Hello,

I have the following code:

from Bio.PDB import Polypeptide

residue
Out[17]: <Residue GDP het=H_GDP resseq=430 icode= >

Polypeptide.is_aa(residue)
Out[18]: True

Why is it True? How can I return True just for standard amino acids?

Regards.
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