Iset not working with Wcf test client.

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WCF

NHibernate

I am Using a Iset Collection in my code.
This gives me an error like this operation not supported in wcf.
I am using wcf test client inorder to test my methods.I am Using Nhibernate in my application to map.

User.hbm.xml

<class name="User" table="user" dynamic-insert="true">
  <id name="Id" column="id">
    <generator class="identity"></generator>
  </id>
  <property name="FirstName" column="firstname" />
  <property name="LastName" column="lastname"  />
  <set  name="RoleList"   table="rolemapping" >
    <key column="id" />
    <many-to-many column="roleid"   class="Roles"  />
  </set>

User.cs

private ISet<Roles> roleList = new HashSet<Roles>();

virtual public  int Id { get; set; }
        virtual public  string FirstName { get; set; }
        virtual public string LastName { get; set; }

virtual public ISet<Roles> RoleList
      {
          get { return roleList; }
          set { roleList = value; }
      }

I want to fetch and insert values from/to user table using a link table role mapping.
I already tried everything with bag,ibag tags.But it didn't worked.
Is there any other way or please help me on this?