Why does StringValidator always fail for custom configuration section?

Question

I have created a custom configuration section in a c# class library by inheriting from ConfigurationSection. I reference the class library in my web application (also c#, ASP.NET), fill in the appropriate attributes and everything works great. The problem starts when I start adding validators.

For example, this property:

    [ConfigurationProperty("appCode", IsRequired = true)]
    public string ApplicationCode
    {
        get
        {
            return (string)base["appCode"];
        }
        set
        {
            base["appCode"] = value;
        }
    }

As is it works fine, but as soon as I add this:

    [StringValidator(MinLength=1)]  

It bombs with the following error:

The value for the property 'appCode' is not valid. The error is: The string must be at least 1 characters long.

I get this error even though a valid appCode value is in my web.config file. If I remove the validator it works perfectly. Does anyone know how to get around this?