control difference track volume at same time with 1 slider

Hi, noob here !
So, I'm trying to do that, same value at the same time for each track volume assigned.
ex : fader 1 = vol tracks 1, 3, 5.
I reach my goal with adding few track selector, but I would like it works with track name, so I tried with reaction.
here is the log message error :
(test reaction loop) There's a problem with 'Action Block 1' in reaction 'Reaction 1' (from 'Slider 1 was moved' listener) >>
>> Invalid value. Check the parameters range with min/max

I tried different things but I failed.

script at the moment :
listener : slider 1 was moved
loop : tracks
action : track - set volume value - number : Slider 1's latest velocity value

What it does at the moment is set all tracks volume alternatively max - min while move fader 1.

What am I missing ?