Compair 2 sets of numbers

I have a business in which I compare my customer scores monthly
Attached is a sheet

Column A is Cust ID and B is Cust score previous moth

Column D is Cust ID and E is Cust Score current moth

Column E is the Movement in score If found

=(E3-(LOOKUP(D3,A:A,(B:B))))


This formula works but if the current customer is not found in Column A
It uses the score in column A that is just under it.
For example say 793 is in the D column current list but not found in column A
But 795 is in column A it will use 795 Score in the computation which is wrong

This formula works in finding customers but the computation doesn’t work

=IF((SUMIF(A:A,D2,B:B))>0,(E2-(SUMIF(A:A,D2,B:B))),0)

Also not sure what I can do if current customer not found maybe put a NULL for score

Help would be appreciated