Getting error while using DefaultViewModel n creating windows 8 metro app

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Silverlight

Metro-design

I have a class

class Strategy
	{
		public Implementataion Implementataion = new Implementataion();
		public BenefitRisk BenefitRisk = new BenefitRisk();
		public Overview Overview = new Overview();
		public string RBId { get; set; }
		public string Name { get; set; }
		public string NameColor { get; set; }
		public string Picture { get; set; }
		public string Id { get; set; }
	}

I am calling a method while returns me an object myStrategy of type Strategy.

While performing the following operation I am getting an exception "Value does not fall within the expected range."

this.DefaultViewModel["Strategy"] = (object) strategy;

Need help.

[Edit]Code blocks added[/Edit]