C# - StringValidator Always Fail For Custom Configuration Section?

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.