Index is not returning -1

Hi,

We need to provide a facility to business user to enter like item (rather than pick from a picklist with all valid items).

We used index function index([prod].[item], likeitem) to find the index of an item entered by user into a measure.

If the user entered an invalid item, I expected the index function to return -1. However its returning index 0.

can anyone please tell me why its returning 0 and not -1 for invalid item?

Regards,

Anusha