extend worker sent an error! Uncaught TypeError: Cannot read properties of undefined (reading 'Callback')

v 3.1.60,v3.1.64

I use this simple example in my project and it works for non pthread building.
https://emscripten.org/docs/porting/connecting_cpp_and_javascript/embind.html#extend-example

But then I compile with pthread I got random (through one or two page reloads) error:

`worker sent an error! https://localhost:4443/wasm.js:71: Uncaught TypeError: Cannot read properties of undefined (reading 'Callback')`

```
const delegate = MODULE.Delegate.extend( "Delegate", {
__construct: function () {
this.__parent.__construct.call(this);
},
__destruct: function () {
this.__parent.__destruct.call(this);
},
Callback: function (jmessage) {
console.log(jmessage);
},
}
);

let d = new delegate();
```

How can I try to figure out what the cause of the error is?