Thread:Semantic-mediawiki.org:Community portal/Querying for non-empty values

Hello,

I'm trying to query those pages to count where a particular property is not empty. I've used as follows:

A number does result indicating how many pages have "Competitor" not null, but an error results as well: "Empty strings are not accepted".

Let me ask, is this the right way to query for non-empty properties? If this is the only possible way to do, is there a way to suppress the error?