[Bioc-devel] class declaration

Dear all,

I am creating a class for my function composed by two expression set:

let say

setClass("my class",  representation(a="ExpressionSet", b="ExpressionSet")

when I lunch it I retrieve this warning.

[1] "my class"
Warning message:
In .completeClassSlots(ClassDef, where) :
  undefined slot classes in definition of "my class": a(class
"ExpressionSet"), b(class "ExpressionSet")

how can  solve this problem?