Count and sum unique Values

[skorpion]

Im trying to get the sum of Sales with uniuqe Catatery im using thus DAX Formula please anyone can help me in this

Countoracle = CALCULATE(SUM('Data Source'[Sales) ,DISTINCTCOUNT ('Data Source'[ItemAr]))

getting error 

The True/False expression does not specify a column. Each True/False expressions used as a table filter expression must refer to exactly one column.