Preventing duplicates with data validation

[DashboardNovice]

http://chandoo.org/wp/2009/10/26/duplicate-data-entry/

I am using the same formula and the same cells as the example but it's not working. As soon as I type a letter or number into cell B4, I get flagged with an error. I don't know what could be wrong. Any ideas?

Note: The COUNTIF formula in the formula field has a semi-colon ( between both arguments. It should be a comma.

 

[Hui]

The use of : or ; is Excel language dependent

Can you please post a sample file

 

[DashboardNovice]

This time I copied and pasted the formula and now it works. I guess I must have had a character off or something. Oh well.