(wasm) data in copyToWasm() can be undefined

[guest271314]

Firefox WebTransport client.

TypeError: Cannot read properties of undefined (reading 'byteLength')
    at copyToWasm (webtransport-example.js:94:12)
    at writeStreamData (webtransport-example.js:305:3)
    at Object.onStreamData (webtransport-example.js:396:7)
    at pollEvents (webtransport-example.js:225:32)
    at webtransport-example.js:492:7
TypeError: Cannot read properties of undefined (reading 'byteLength')
    at copyToWasm (webtransport-example.js:94:12)
    at writeStreamData (webtransport-example.js:305:3)
    at Object.onStreamData (webtransport-example.js:396:7)
    at pollEvents (webtransport-example.js:225:32)
    at webtransport-example.js:508:7

I've done

if (data === void 0 || data.byteLength === 0) {
    return fn(0, 0);
  }

though I don't think data is ever expected to be undefined?

Additionally, if I send 1 MiB to Firefox client Firefox never completes the roundtrip, and I think retains the connection and data after connection lost.