<Shift+Action Button> bindings not working

If I bind an action button to a key with no modifiers, like <1>, <shift+1> will not activate that button. It highlights when I activate it with shift, but no action fires.

The action button triggers fine if <alt> or <ctrl> are held, and even a combination of <alt|ctrl+shift> works. No matter the non-modifier key I used for the binding, I could not activate it with the <shift> key as the only modifier.