String was not recognized as a valid DateTime

I want to add a date in session (date1) like this:

Session["DateLesson"] = date1.ToString("dd.MM.yyyy");
 

Now from the session I want take this value:

var asd = Session["DateLesson"];
 /*asd = "20.04.2012"*/
 var datelesson = DateTime.Parse((string) asd);
 

And it gives me this exception:

FormatException not recognized as a valid DateTime

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