WEEKNUM Not recognizing Date Column

I have created a simple date table and I am trying to add week number to the columns.  However I am getting the error message below.

Here is the date colum.  It is formated as a date.

And here is the error message.

"A single value for column 'Date' in table 'dcalendar' cannot be determined. This can happen when a measure formula refers to a column that contains many values without specifying an aggregation such as min, max, count, or sum to get a single result."

Any Ideas why this isnt working?

Thank you,

Joseph