c# - DynamicObject's property needs to be casted before invoking extension methods?

Question

I created a class that extends DynamicObject:

public class DynamicEntity : DynamicObject
{
    private readonly Dictionary<string, object> values = new Dictionary<string, object>();

    public override bool TryGetMember(GetMemberBinder binder, out object result)
    {
        values.TryGetValue(binder.Name, out result);
        return true;
    }

    public override bool TrySetMember(SetMemberBinder binder, object value)
    {
        values[binder.Name] = value;
        return true;
    }
}

I can add property to the instance of my class. For example:

dynamic person = new DynamicEntity();
person.firstName = "John";
person.birthDate = "January 2, 1990";

I can also use methods and properties of string:

Console.WriteLine(person.firstName.Length);
Console.WriteLine(person.firstName.Contains("ohn"));

However, using extension method,

bool empty = person.firstName.IsEmpty();

gives me an error 'string' does not contain a definition for 'IsEmpty'.

The error can only be suppressed when I cast the property to string:

bool empty = ((string)person.firstName).IsEmpty();

I wonder why can I use string methods without casting while I cannot use extension methods without casting.

Any idea why can't I use extension methods without casting?