Module formula field Output zero if one of the fields in the formula is blank

Module formula field Output zero if one of the fields in the formula is blank

Hi All, 

Hoping this is an easy one for someone to answer, I have the following calculation in a formula field

(${Products.Trade Price}-${Products.Wholesale Price}) 


This works fine but if the Wholesale Price is not set I need the output to be zero.  At the moment, understandably,  it is showing the total as the Trade Price if Wholesale is blank.


Scratching my head on this one...

Tried the likes of this 

IF(ISNULL(${Products.Trade Price}) OR ISNULL(${Products.Wholesale Price}), 0, ${Products.Trade Price} - ${Products.Wholesale Price})

but getting syntax errors