Frequency Function Issue

Good Morning Everyone,

Not sure if the Frequency function supports what i am trying to achieve, and i appreciate that there is an easier way to get a count of unigue numbers using other functions, but trying to understand why Frequency wont support this formula.

What i am trying to achieve is get a unique count from a list of P/O numbers where there are lots of duplicates as a P/O No. could be specific to a single line item or have multiple line entries.

Now first if the formula that works provided i enter as an array using control + shift + enter

=SUM(IF(FREQUENCY(MATCH(B7:B1050,B7:B1050,0),MATCH(B7:B1050,B7:B1050,0))>0,1))

However, when i enter the following formula using structured references as my data is embedded in a table i get the #N/A response if entered normally or as an array.

=SUM(IF(FREQUENCY(MATCH(Table1[[#Headers],[PO Number (Header)]:[PO Number (Header)]],0),MATCH(Table1[[#Headers],[PO Number (Header)]:[PO Number (Header)]],0))>0,1))

So does the Frequency function not work with structured references?, if that is the case good to know for my onward excel learning path.

Thanks in anticipation

Richard.