Return a dimension from expression(set analysis)

[#1]

I'm trying to create a dimension for customer numbers that did not have any sales the previous year.  I'm new to Qlik so please excuse me if this is a stupid question. 

Customer Number	CY Sales	PY Sales
1	1000	20000
2	1000000	100000
3	2000	0
4	300000	0

I tried using an if statement, but it said that it was an invalid dimension. Here's the if statement:

if(sum({<[Year]={$(=max([Year])-1)}>} EXTSHIPAMT)=0,CUSTNUM)

I'm assuming there's a function that does this, I'm just not aware of it.

Any help is appreciated!

Thanks,

James

[#2]

It still summed up the null values. 

[sunny_talwar]

What if you try this for me:

Sum({<[Year]={$(=max(Year)-1)}, [New Account] = {"=Len(Trim([New Account])) > 0"}>} If(Aggr(Sum({<[Year]={$(=max(Year)-1)}>} If(STATUS=31,-1*EXTSHIPAMT,EXTSHIPAMT)),[Cust Name])=0, 0, If(STATUS=31,-1*EXTSHIPAMT,EXTSHIPAMT)))

[#4]

Same thing.

[sunny_talwar]

You won't be able to share a sample?

[#6]

I'm actually not sure how to share a sample.