custom parameter attributes

Custom parameter attributes aren't being added.

import System
[AttributeUsage(AttributeTargets.Parameter, AllowMultiple: false, Inherited:true)]
class DemoAttribute(Attribute):
def constructor():
print "DemoAttribute constructor"

class DemoClass:
def DemoMethod([Demo] demoArg):
pass

print typeof(DemoClass).GetMethod("DemoMethod").GetParameters()[0].GetCustomAttributes(true)[0]