<null> generated with replace item doesn't work

Hi guys

I am trying to compute the angle between 2 list of lines

the main difference is that to achieve same lists lenght one of the lists has been filled with a few "fake" <null> ( text )

Now : when i compute the angle between the 2 lists the "fake" null is giving me trouble.

does anyone know how to fix it ? i noticed a clear visual difference between the "fake" <null>  and the real one.

This last is infact faded while the other one has a different look

thanks in advance for your help

Ivan