Hi @Fed17,
Your very welcome, glad it helped you out. 
The issue you have now is that you are removing the null values. As I said above, the two lists need to be the same size. Basically your filter mask node retained the blank value at index 0 because your mask value at index 0 was true
So to get around this, you need to instead replace the nulls with False Boolean values. As false represents the data you wish to filter out.
See example method below.
The “List.ReplaceNull” is from the Clockwork Package 
Best
Ben