bug#56682: locked narrowing

[Gregory Heytings]

> > I don't think I understand (you avoid the assertion in other cases 
> > which look similar to me), but if there are good reasons, please add 
> > comments there explaining the rationale.
>
> I'm just being extra cautious here.  I added a comment to explain that 
> assertion (and the previous one).

In the end, I removed that assertion, there are indeed no "good reasons" 
to keep it.