DirectorySearcher.FindOne throws exception when search string is not formatted properly.

I specified Searcher.Filter = ((objectClass=user)(SAMAccountName=09433923)).
I forgot to add an & before objectClass Filter.
I kept getting "The directory service is unavailable" exception when Searcher.FindOne() method was called.
As soon as I added the & and set the searcher to Search.Filter = (&(objectClass=user)(SAMAccountName=09433923)) , it worked.
I did not expect an exception to happen if the search string was invalid.
I was expecting a null result potentially.