".Find" Returns Run-time error '91': if the Item Is Not Found.

I am trying to establish whether or not a Bank Holiday date appears on a list so I can use Workday_Intl to obtain the date of the previous working day.

I am using the code line

BankHoliday = .Find(what:=FirstMonday, LookIn:=xlValues, LookAt:=xlWhole)

which works fine when the search date is on the list but displays Run-time error '91': if it is not. I have to say this must take the award for being the most obscure, meaningless, and misleading error message of all time.

What use is .Find if it cannot be used to find whether or not something is present?

I have attached an example spreadsheet demonstrating the problem.

Any help will be much appreciated.
Tegglet