IF (True/False, TopN1, TopN2) get error message

I need get Top N value from Slicer, but some slicer value is +, some slicer is -,  when slicer is +, I will choose TopN, when slicer is -, I will choose bottom N, here is my DAX : 

TopN = 

var TopProductDesc = TOPN(25, ALLSELECTED('Ultimate'[Credit Ultimate]), [DateSale], DESC)

var TopProductAsc   = TOPN(25, ALLSELECTED('Ultimate'[Credit Ultimate]), [DateSale], ASC)

var AscorDesc = SELECTEDVALUE('Measure'[Measure]) IN {"A","B","C","D"}

var TopProduct = IF ( AscorDesc , TopProductDesc, TopProductAsc)

RETURN

TopProduct

error message is :

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

If I want according measure decide TopN desc or TopN asc, How to modify my DAX, Thank you for help