Edited Issue: "An item with the same key has already been added." exception [358]

I get the above exception when I try to use the [Queryable] attribute with the following model:

public class Store
{
public Store()
{
this.Municipality = new Municipality();
}

public string Id { get; set; }

public string OrganizationNumber { get; set; }

public string Name { get; set; }

public string Department { get; set; }

public string PhoneNumber { get; set; }

public string EmailAddress { get; set; }

public string Website { get; set; }

public string LogoPath { get; set; }

public string PicturePath { get; set; }

public IEnumerable<Tag> Tags { get; set; }

public IEnumerable<Industry> Industries { get; set; }

public Municipality Municipality { get; set; }

public string Address { get; set; }

public PostalAddress PostalAddress { get; set; }

public GeoCoordinate Coordinates { get; set; }

public string FloorId { get; set; }

public Position Position { get; set; }

public string Description { get; set; }

public string SecondaryDescription { get; set; }

public IEnumerable<OpeningHour> OpeningHours { get; set; }

public string FacebookPage { get; set; }
}

The Municipality class has a property named State of type State, and PostalAddress has the same property.
Could that be the problem?

All the other complex types are "flat", meaning they only contain string, int, etc.