Searching mails with blank titles

Hello,

We are using Discovery Accelerator 11 and one of the specific search we intend to do is to look for all the mails with a blank subject.

In the search terms, for "Subject"  we've tried to put the entry "" or to press space. But the system don't understand this command.

Does someone knows how to do that? I've searched on the other posts but can't find someone else having the same problem.

 

Thanks and regards