Aggregation Problem in Webi

HI,

I have the following:

Contract1

Material 1     Factor1.1

Material 2    Factor 1.2
Material3    Factor 1.3

Contract 2

Material 1 Factor 1.1

Material 2  Factor 1.2

Material 3 Factor 1.3

Contract 2

Material1 Factor 1.1

Material1 Factor 1.1

Material1 Factor 1.1

Material2 Factor1.2

I am trying to write a formula to do:

sum(Material*Factor)for each(Contract)

It is working only for Contract 1 and Contract 2. Giving a multivalue error on Contract 3. Any suggestions? Please let me know.

Thanks

SriG