Get Value Type from ConstantExpression when Value is null

How can I determine the type of a ConstantExpression Value when it is null? I have previously been detecting the type using the following code, but it causes a null exception when the ConstantExpression Value is null.

static Type GetType(Expression expression)
{
    //causes exception when ((ConstantExpression)expression).Value is null
    if (expression is ConstantExpression)
        return ((ConstantExpression)expression).Value.GetType(); 

    //Check other types
}

Imagine my Expression was created similar to this:-

int? value = null;
ConstantExpression expression = Expression.Constant(value);

And I would like to determine the type int?