SOLO transaction requests returning null

Howdy! We've been messing around with token integration for our project. Had a smashing success integrating STX into our system, however, i am bumping into a few issues with some other tokens, namely SOLO.

A few things I can scratch off as possible causes are things like tokens, addresses, issuers etc, not only because the system I have built works well with a different token (STX) but also because, had this been the case, usually the Xumm package does a good job at returning a descriptive error of what went wrong, however, when i try to create this request, I am just getting a null response from the Xumm SDK which gives me very little to work with.

This is an example request which is returning null.

I have purposefully skipped the account and destination fields as previously explained.

{
  options: { expire: 5 },
  custom_meta: {
    instruction: 'Thanks for using [Our Product]. Please approve your SOLO transaction.'
  },
  txjson: {
    TransactionType: 'Payment',
    Account: '-',
    Destination: '-',
    Amount: {
      currency: 'SOLO',
      issuer: 'rsoLo2S1kiGeCcn6hCUXVrCpGMWLrRrLZz',
      value: 40
    }
  },
  user_token: '-'
}

For comparison, a different request with the currency set to STX and with the right issuer address for that works just fine, however, when I submit this using xumm.payload.create(request) I just get a null response.

We're currently using version ^1.6.5of the xumm npm package.

Thanks in advance for your help!