Why does casting int to invalid enum value NOT throw exception?

If I have an enum like so:

enum Beer
{
    Bud = 10,
    Stella = 20,
    Unknown
}

Why does it not throw an exception when casting an int that is outside of these values to a type of Beer?

For example the following code doesn't throw an exception, it outputs '50' to the console:

int i = 50;
var b = (Beer) i;

Console.WriteLine(b.ToString());

I find this strange...can anyone clarify?