Simple Dimension Formula

Hi All,

I am trying to divide two accounts where AccountB is from prior year end

AccountA

AccountB

KPI = ([Accounts].[AccountA]/([Accounts].[AccountB],[Time].Currentmember.Properties('BEGIN')))

However this fails to validate during processing at expanding formulas and indicates incorrect syntax (actually It never gets to OLAP processing)

Please let me know if anyone has any ideas.