Can someone please tell me why this function does not work:

SUM(IF(E199>"0",SUM(E199-D199)))

In case I have it all wrong, if I have a Value in cell E199 and I want to
subtract E199 from D199 provided that E199 has a value in it, if it does not
then I want Cell F199 where my sum is to remain blank.

Hope someone can help me with this.

Many thanks in advance

Stu