Validating certain parts of input string using Fluent Validation

I am using `ASP.NET 4` and `Fluent Validation`. I am trying to setup a rule that checks if my user name starts with "adm". I have the following, giving me errors. I tried to follow the online sample but it is not working: ``` RuleFor(x => x.UserName) .NotNull() .WithMessage("Required") .Must(x => x.UserName.StartsWith("adm")) .WithMessage("Must start with ADM"); ``` I don't think I am doing it correctly? I've event tried: ``` .Must(x => x.UserName.Substring(0, 3) == "adm") ``` Error for both: ``` Delegate 'System.Func<MyProject.ViewModels.LogOnViewModel,string,bool>' does not take 1 arguments ```