TypeError: "challenge" must be a string (ProofPurpose).

[lemoustachiste]

According to the data integrity spec https://www.w3.org/TR/vc-data-integrity/#verify-proof, the challenge parameter is optional.

However I'm getting the following (non-blocking Actually, this is the (first) failing error, not the fact that proof proofPurpose does not match the purpose) error when I haven't specified it:

TypeError: "challenge" must be a string.
at new AuthenticationProofPurpose (.../node_modules/jsonld-signatures/lib/purposes/AuthenticationProofPurpose.js:15:13)