Using git master as of today, Vala will generate a coroutine state value that is unreachable from the internal jump table.  I will attach the full file, but this is the relevant part:

    public async void query_async() throws Error {
        running = true;
        try {
            if (!yield internal_query_async())
                return;
        } finally {
            try {
                yield close_query_async();
            } catch (Error err) {
                // ignored
            }
            
            running = false;
        }
    }

In order to reproduce this, it appears that (a) there has to be a code path within in the try block that will exit the method, and (b) there has to be code executed inside the finally block after the async call there.

The generated C code will set the internal _state_ field to a value that is not found in the _co function's jump table.  This happens before calling close_query_async().  That in turn leads to a "code should not be reached" hard assertion when it completes.