Set a calculated parameter not working

I am trying to set a parameter value from a formulation to only some of my walls (those containing SEINÄT in its name) and at the Element.SetParameterByName node I’m getting the message:
Warning: Internal error, please report: Dereferencing a non-pointer.
I have tried to set this parameter to all the walls and it’s working so it seems to me that the error is coming from the input I am giving to this last node. Should it be an element as input and not a string? How can I get back to the element from my desired list of walls?
I am using revit 2016 and dynamo 1.1
I hope someone can help me as it should be something quite simple, thanks!

You are now trying to set the Parameter of a Category name. If you connect the output of the All Elements of Category-node to the List.FilterByBoolMask - list input it should work.

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Thank you very much Brian! It is now working perfect