Formula Checkbox based on User Field Present

I am trying to write a formula checkbox formula that would be checked when a user field has a user listed.  I tried a couple different formulas but couldn't get it to work. I think I am missing something....

[Employee-Previous Supervisor] is my user field

1st try- Case([Employee - Previous Supervisor],null,false,true)

2nd try- If([Employee - Previous Supervisor]<>null,false,true)


What am I missing. Do I need to covert my user field to something else?

Thanks,

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Ryan Whitener
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