Expression vs Func compile time error

packages.config

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="FakeItEasy" version="4.5.1" targetFramework="net461" />
  <package id="Mandrill" version="2.4.1.0" targetFramework="net461" />
  <package id="Newtonsoft.Json" version="6.0.6" targetFramework="net461" />
</packages>

Program.cs

using FakeItEasy;
using Mandrill;
using Mandrill.Requests.Messages;

namespace ConsoleApp2
{
    class Program
    {
        static void Main(string[] args)
        {
            var mandrill = A.Fake<IMandrillApi>();

            A.CallTo(() => mandrill.SendMessageTemplate(A<SendMessageTemplateRequest>.That.Matches(
                    x => x.Message.GlobalMergeVars[0].Content == 0, "testing"))).MustHaveHappened();
        }
    }
}