Using index match offset to find first date that amounts drop to zero

Hi,

Trying to build a formula that returns the first date that a series of amounts ceases, ie drops to zero, using an index on date range, and attempting to match where amount is zero and amount of month prior is >0.

Importantly, the series of payments may not begin in first month, so no value in just finding the first zero amount...

My formula currently is CSE but returning #n/a:

{=INDEX(C1:F1,,MATCH(TRUE,(C2:F2=0)*(OFFSET(C2:F2,0,-1)>0),0))}

Cells B1..F1 are a range of 5 dates from 1 Jan to 1 May

Cells B2..F2 are amounts 0,0,500,500,0

In the above case formula should be returning "1 May", as when series first dropped to zero.

Many thanks in advance...