[luatex] this can't happen error with Umiddle (luatex 1.07/texlive 2018)

[David Carlisle]

The following plain tex example


$\left( A \middle|  \right)$ %OK



$\Uleft( A \Umiddle class 2 | B \Uright)$ %OK


$\Uleft( A \Umiddle class 2 | \Uright)$ %oops

\bye

produces


! This can't happen (mathspacing).
l.10 $\Uleft( A \Umiddle class 2 | \Uright)$
                                           %oops
!  ==> Fatal error occurred, no output PDF file produced!



I'd actually meant to have the A|B but the error message for my typo
seemed a bit harsh!

Having an empty math list to the right of a class 2  \Umiddle might be
odd but shouldn't be an error I assume?

David