Keywrap bug using `SymmetricKey.generate()`

[grempe]

If using the instructions provided in the README for Keyring Keywrap it fails with:

    let wrap = await SymmetricKey.generate();
    
    // alternatively, if keywrap key is set to null it works
    // let wrap = null

    let ring = new Keyring(wrap);
    let symmetric = await SymmetricKey.generate();
    let key = await ring.save(symmetric);

TypeError: Invalid type; string or buffer expected

See the following failing runkit example:

https://runkit.com/grempe/604bb5107fed34001a98c583