Facing issue during verifying BBS Verifiable Presentation

[projjal-gop]

This is the code to verify the VP using derived proof.
const verification = await vcjs.verifyVerifiablePresentation({ presentation: verifiablePresentation, challenge: 'nonce-123', domain: 'example.com', suite: [new BbsBlsSignature2020(), new BbsBlsSignatureProof2020()], documentLoader });

I have written custom documentLoader to fetch context document as well as resolve did:key:z5t using bls keypair.
But at the time of verification I am getting error
"TypeError: The \"url\" parameter must be a string.\n at JsonLdDocumentLoader.documentLoader

I understand that the verification library is trying to call documentLoader using object which is not string.
Is there something I need to correct in the code or is it something from library end ?