creating expression with a script

Hello all.
I am trying to create a float_expression or a float_script with a script. I want to connect the inner amount of a shell modifier to a custom attribute (called "range" in a Custom Attribute modifier. here is what I do:

obj = $Plane02
ctrl = float_expression()
obj.modifiers[#Shell].innerAmount.controller = ctrl
ctrl.AddScalarTarget "range01" $Point01.modifiers[#Attribute_Holder].Custom_Attributes.range

the 4th line gives me the following error:
-- Runtime error: IExprCtrl::AddScalarTarget - Target needs to be controller or subAnim, got:  11.0
11.0 happens to be the current value of the "range" attribute.

Exactly the same happens when i try to create a float_script like this:

 cntrl = float_script()
obj.modifiers[#Shell].innerAmount.controller = cntrl
cntrl.AddTarget "range01" $Point01.modifiers[#Attribute_Holder].Custom_Attributes.range

only to get the same error... any idea????

update: the expression that i try to create is :
min ((range01/length(objPos - PointAPos) + range02/length(objPos - PointBPos)),2)
however i can not create the necessary variables

Thanks