Multiple selection on slicer not possible for publish app

Hi,

I published a report which the report have a slicer that have options of allowing multiple selections. The options are (blank), closed, completed, following-up. My understanding is by holding the cltr key, multiple selections are allowable. In my case, when user select (blank), multiple selection will be disabled. 

May I know how to fix this? Please advice.

I have a copy of the report on the desktop which is working fine.

Regards

Jeremythe options of the slicer