Conditional Format on 2nd drop down menu - Invalid option

I have two drop down menus, the 2nd menu is an indirect formula on the 1st menu. Let's say Drop down 1 is selected and then 2 is selected, then the user decides to change drop down 1, the option in 2 is no longer valid.

How do I do a conditional format on drop down 2?

This is what I currently have but does not work:
Conditional Format in F16; =indirect($D$16)<>$F$16
D16 = Drop down 1
F16 = Drop down 2

*I already have a VBA which auto clears out the 2nd drop down if the 1st one is changed, however not every user has the newest Excel*

Thanks