Error says Field cannot be Unique

I have a formula that I am getting an error on. The error says: This formula is incompatible with this field being marked unique. Please change the formula or uncheck the unique checkbox.

This is my formula

[Invoice #]&" - "&[Subcontractor/Supplier]

I am combining two fields so that I can require the field to be unique. 

What am I missing? 

Thank you,

Brandy

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Brandy Nunes
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