Extracting Part of Field

Hi,

I want to extract all data to the right where the field contains 'User:' and to include User:

I am using this formula which works great.

=Right([Opportunity Description];(Length([Opportunity Description])-Pos([Opportunity Description];"User:")+1))

However when the field has data in but not 'User:' it brings back the whole field, I just want this to be blank.

Any ideas?

Thanks,

Gwyn