Control if all rooms in a view have room tags

Hello everybody! I wanna make a definition which looks into a view and controls if all rooms have room tags. My idea is collects all rooms in a view (list a) , collects all room tags in the same view (list b).

The definition works well when i use “All Elements in active view”


. , room tag 02 The error reads : Warning: List.FilterByBoolMask operation failed. Value cannot be null. Parameter name: source Any ideas ? Thank Shahrokh