Scripting Problem Custom Attributes

I am new at scripting and I've been trying to make a test script that would add custom attribute to an attribute holder modifier.

This is my script:

ca=attributes Attritest
(
parameters params rollout:Testbox
(
boxheight type:#float boxheigthsp
)
Rollout Testbox "The test of the box"
(
spinner boxheigthsp "MY HEIGHT!!" range: [0,100,0]
)
)

custattributes.add $.modifiers[1] ca

And this is the error I get:

-- Error occurred in anonymous codeblock; filename: I:\Encour\testbox.ms; position: 252; line: 13
-- Type error: custAttributes.add requires MSCustAttribDef, got: undefined

Thank you in advance for help