Assigning Keyboard Shortcut to "Collapse All" doesn't work

When I assign a keyboard shortcut to "Collapse All" in Project Explorer, it has no effect.
It looks like it should work, but doesn't in practice. I have tried a number of different key combinations, but to no avail.

(Where I work, we have a a large source tree with many branches, and expanding and collapsing contributes a significant part of my working day.

Being able to Collapse everything in Project Explorer via the keyboard would be very welcome functionality indeed.)

Thank You.