In src/browser/transport.js:

This line recently changed from var someOtherVariable = to var payload = .

payload is already defined in the function definition (it's spread out from the first argument). Redefining with var is technically correct but it's tripping up Turbopack because Turbopack is transforming it into this code in dev:

Transport.prototype.post = function(param) {
    let { accessToken, options, payload, headers, callback, requestFactory } = param;
    /// <...snip...>
    var payload = stringifyResult.value;

Turbopack would be happier if you removed the var which I think would be valid regardless of transformation.

I'm also going to raise this with Turbopack!