How do I create a specimen builder in AutoFixture to always return a string that matches a Regex?

I am trying to use conventions in my unit tests which make use of AutoFixture. I have a Password value object as shown:

public class Password : SemanticType<string>
{
    private int _daysPasswordValid;

    public Password(string password) : base(IsValid, password)
    {
        Guard.NotNullOrEmpty(() => password, password);
        Guard.IsValid(() => password, password, IsValid, "Invalid Password");

        DateCreated = DateTime.Now;
    }

    static bool IsValid(string candidate)
    {
        //password cannot contain be whitespace
        if (string.IsNullOrWhiteSpace(candidate))
            return false;

        const string passwordPattern = @"^(?=.*[A-Z].*[A-Z])(?=.*[!@#$&=()-*])(?=.*[0-9].*[0-9])(?=.*[a-z].*[a-z].*[a-z]).{20}$";

        var match = Regex.Match(candidate, passwordPattern);

        return match.Success;
    }

    public DateTime DateCreated { get; private set; }

    public int DaysPasswordValid
    {
        get { return _daysPasswordValid; }
        set
        {
            const int minimunDays = 0;
            const int maximumDays = 90;

            if (value <= maximumDays && value > minimunDays)
                _daysPasswordValid = value;
            else
                throw new ArgumentOutOfRangeException(
                    "Password must be valid more than {0} and no more than {1} days.".Fmt(minimunDays, maximumDays));
        }
    }

    public static implicit operator string(Password password)
    {
        return password.Value;
    }

    public static explicit operator Password(string value)
    {
        return new Password(value);
    }
}

I would like to be able to create a fixture based on the Password class and have the fixture created with a valid password, in other words, a password that matches the RegEx pattern in the Password class. I have been looking at the RegularExpressionGenerator available in AutoFixture but haven't quite gotten things to work. Here is the builder have so far:

public class ValidPasswordBuilder : ISpecimenBuilder
{
    public ValidPasswordBuilder(string regularExpressionRequest)
    {
        PasswordRegularExpression = regularExpressionRequest;
    }

    public string PasswordRegularExpression { get; private set; }

    public object Create(object request, ISpecimenContext context)
    {
        var pi = request as ParameterInfo;
        if (pi != null && pi.Name == "password" && pi.ParameterType == typeof(string))
        {
            var generator = new RegularExpressionGenerator();
            var regExRequest = new RegularExpressionRequest(PasswordRegularExpression);
            var result = generator.Create(regExRequest, context);
            return result.ToString();
        }

        return new NoSpecimen(request);
    }
}

Here is the test:

[Fact]
    public void ValidPasswordMatches()
    {
        var fixture = new Fixture();
        fixture.Customizations.Add(new ValidPasswordBuilder(PasswordPattern));
        Action a = () => fixture.Create<Password>();
        a.ShouldNotThrow<ArgumentException>();
    }

I can run the test as setup above, and it passes, but if I debug it, I get an error in the IsValid function in the Password class (through the Guard clause) that says that the password returned from the builder is:

Ploeh.AutoFixture.Kernel.NoSpecimen

I never seem to get a result that matches the RegEx pattern. I feel like I'm close, but need some help getting over the hump.

Thanks!