I tried to update my project to the latest version of dmd and vibe and received this error.

Tested with dmd 2.065 and 2.066 on Windows 7 x86 and x64.
Error occurs in vibe >=0.7.21-rc.3 (tested rc.2, rc.3, rc.4, master), driver win32 with code 

void main()
{
    void func(Json c)
    {
        writeln(c);
    }

    Json config = Json(["id": Json(1001)]);
    runTask(&func, config);
    runEventLoop();
}

and that output:
undefined

When changing line 

runTask(&func, config.id);

then output
Task terminated with uncaught exception: Got JSON of type int_, expected object.

Last error occurs ever if clear func() 

void func(Json c){}