Problem Implementing Strongly-typed Profile class in Web application project with VS2008

I can get everything work if I am using ProfileBase
Profile.SetPropertyValue() and Profile.GetPropertyValue().


But if I inherit the ProfileBase to make a strongly-typed class and return it to the page like this
protected CustomProfile Profile
        {
            get
            {
                return (CustomProfile)HttpContext.Current.Profile;
            }
        }
It will give me an invalid cast exception.
What should I do? no one mention this exception when they implement like this.

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