Index functions inside conditional IF

Hi,

I have a rule like this, where I am trying to fetch a date value from an index:

Meas      base_intx      type              NA Value

A             store            Integer          99999

B             store            Date

Meas 'A', is assigned a valid index of date based on some conditions, else it has the NA value (99999).

Meas B is calculated as:

B = If (A!=99999, indextostartdate (A,[clnd].[day]), ignore)

Now, when this expression is executed, it gives the following error: 'Calendar Hierarchy cannot access position 99999 in dimension Day (Size is 1736)'

This is because, we have only 1736 positions in calendar (history and future) and I understand this.