Expression Error IF is Then else

Hello Everybody, 

i have a probem in writing an if function with power query. 

I have table with "delivery date wish customer" (gepl.Versanddatum) and "delivery date possible" (machbarer Termin)

Problem is that it could be that the "delivery date possible" is a empty cell because the date is equal to "delivery date wish customer".

Therefore i wrote the following formula with the formula editor from "add customer column"

if [#"xmr_vk_analyse (2).Machbarer Termin"] is null then Date.WeekOfYear [#"xmr_vk_analyse (2).gepl. Versanddatum"] else Date.WeekOfYear [#"xmr_vk_analyse (2).Machbarer Termin"]

This leads to the error message shon in the picture at the end of this contribution.

What is wrong with this formula? 

Thanks in advance for helping.