Sum variable number of columns in same row, but avoiding circular reference error

Hello all.

This is my first post, and I'm a newcomer to Excel!

I have tried to write a function to:

• dynamically sum (ie the sum updates as additional columns of values typed) all values rightward of a certain cell along one row - D2 in this case.
• If some of the intermediate cells along that row are blank, these need to be treated as having a value = zero

However I can't figure out how to break out of the resulting circular reference.

Here's the function in question, and the .xlsx is attached.
=SUM(OFFSET(D2,0,0,1,COUNTA(2:2)))