The simple code that follows shows two different problems detailed below.

using System;
using System.Linq.Expressions;

class Test
{
    public static void Main()
    {
        int? i = 0;
        Expression<Func<String>> e3 = Expression.Lambda<Func<String>>(
            Expression.Convert(Expression.Constant(i), typeof(String),
                               typeof(Convert).GetMethod("ToString", new Type[1] { typeof(int?) })));
        Console.WriteLine("SHOULD NOT WHIS BE NULL? " + typeof(Convert).GetMethod("ToString", new Type[1] { typeof(int?) }));
        Console.WriteLine(e3.Compile()());
    }
}

Problems:

While Convert does not have a ToString(Int32?) method, GetMethod returns a method with ToString(Object) as signature instead of null.

The program fails during Compile() with a "System.InvalidProgramException: Invalid IL code" exception.