I've created a TestSet named 'TS-1' in a '\\Root\\TestFolder'

In TestLab, I try to listed out all the test cases that not yet included in TS-1 yet.

So in TestPlan tree pane (on the right of the Window), I try to create a filter which have
TestSet: Not TS-1

But the end result listed out all the test cases (those included and not included in TS-1)

PS: If I set TestSet: TS-1
Then it only listed out test cases has been assigned to TS-1

How can I achieve my intended result by playing around with the filter?

Thanks in advance.