OData v4 Expanding Multiple One-Many - rest

Question

I'm creating an ASP.NET Core 3.1 Web API using OData v4.
I just made a GitHub repo here containing the entire solution, even the database project with dummy data.
Some blogs helped me along the way:
Experimenting with OData in ASP.NET Core 3.1
Supercharging ASP.NET Core API with OData
I've successfully created 3 basic endpoints that can be queried (Countries, Cities and Customers).
The Country and City endpoints work as expected, it is the Customer endpoint that causes some issues on $expand.
The Customer model looks like this (please note that I am currently using domain entities instead of DTO's because I want to get everything working smoothly first, before projecting them to DTO's):
public abstract class AppEntity : IAppEntity
{
[Key]
public int Id { get; set; }
}
public class Customer : AppEntity
{
public string Name { get; set; }
public string AddressLine1 { get; set; }
public string AddressLine2 { get; set; }
public virtual City City { get; set; }
public string VAT { get; set; }
public virtual List<CustomerEmailAddress> EmailAddresses { get; set; }
public virtual List<CustomerNote> Notes { get; set; }
}
With the following models acting as navigation properties:
public class CustomerEmailAddress : AppEntity
{
public Customer Customer { get; set; }
public string EmailAddress { get; set; }
public bool IsPrimary { get; set; }
}
public class CustomerNote : AppEntity
{
public Customer Customer { get; set; }
public DateTime DateTime { get; set; }
public string Message { get; set; }
}
Most of my queries are successful:
Just the collection: https://localhost:44309/api/customer
Expanding the City: https://localhost:44309/api/customer?$expand=City
On of the one-many relationships: https://localhost:44309/api/customer?$expand=Notes
But as soon as I try to expand 2 or more one-many properties or expand all (?$expand=*), I get an exception:
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. (Parameter 'index')
Any clue where this exception might be coming from?
My EdmModel is defined as:
IEdmModel GetEdmModel()
{
var odataBuilder = new ODataConventionModelBuilder();
odataBuilder.EntitySet<Country>("Country");
odataBuilder.EntitySet<City>("City");
odataBuilder.EntitySet<Customer>("Customer");
return odataBuilder.GetEdmModel();
}