Using modifiers not working as expected

Code:

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#showtooltip
/cast [mod:alt,@player] Renew

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I put this on a button and bind ALT+1 to it.
If I press ALT+1, nothing happens. <-- This is what I'm trying to get to work.

If I hold down ALT, the icon changes to the Renew icon.
If I hold down ALT and click the button, Renew is cast on me.
When I press ALT+1, the button actually 'flashes' like it was activated, but nothing happens.

I've verified that the button is the only thing with ALT+1 bound to it (look in WoW keybinding dialog + all my bindings-cache.wtf files).
I've turned off 'Auto Cast Self' in options as well (which was bound to ALT).
I'm using Bartender4 and have turned off the "Self-Cast by modifier" and "Focus-Cast by modifier" options.

Why is ALT+1, when pressed, not casting Renew on myself?
Is the MOD option not supported for keybinds and only when using the mouse?

It appears that having ALT+1 bound to the button was causing the issue.

I removed the ALT+1 binding, bound '1' to the button and then pressed ALT+1. That worked.