Difficult node.js + libuv + v8 error

I am doing some parallel work with uv_queue_work in a node.js c++ addon.

Everything works fine until I try to callback to JS from the Complete method.

I get: CHECK(thread_local_top()->TryCatchHandler() == that) failed.

Any pointers as to what could be causing this?

I am using the following code to callback:

    //Get emit method from JS object and check !IsEmpty and IsFunction
    v8::TryCatch tc;                
    emit->Call(pxy,1,argv);   // The error happens in this statement.
    if (tc.HasCaught()){
        //Some handler code
    }