EF 6.1.3 Multiplicity conflicts - entity-framework

Question

I have been getting error when I try to run add-migration script.
The error is:
Domain.DataAccessLayer.AllRoutines_Product: : Multiplicity conflicts with the referential constraint in Role 'AllRoutines_Product_Target' in relationship 'AllRoutines_Product'. Because all of the properties in the Dependent Role are non-nullable, multiplicity of the Principal Role must be '1'.
And I cannot figure what I am doing wrong. I have AllRoutines and Product entities. AllRoutines can have 0 or 1 Products. Here is my AllRoutines class (some code has been omitted for clarity):
public class AllRoutines
{
[Key]
public long Id { get; set; }
public string Name { get; set; }
public string Tags { get; set; }
public int? RoutineLevelId { get; set; }
public RoutineLevel RoutineLevel { get; set; }
public Guid? ProductId { get; set; }
public virtual Product Product { get; set; }
}
Here is FluetnApi mapping (again some code is omitted):
public void MapRoutine(DbModelBuilder modelBuilder)
{
modelBuilder.Entity<RoutineLevel>().HasKey(r => r.RoutineLevelId);
modelBuilder.Entity<AllRoutines>()
.HasKey(r => r.Id)
.Property(r => r.Id)
.HasDatabaseGeneratedOption(DatabaseGeneratedOption.Identity);
modelBuilder.Entity<AllRoutines>()
.HasOptional(r => r.RoutineLevel)
.WithMany()
.HasForeignKey(r => r.RoutineLevelId);
modelBuilder.Entity<AllRoutines>().HasOptional(c => c.Product)
.WithMany()
.HasForeignKey(c => c.ProductId)
.WillCascadeOnDelete(false);
}
Also I am not sure if this is important or not, but there is also class CustomRoutine which inherits AllRoutines and looks like this:
public class CustomRoutine : AllRoutines
{
public DateTime DateCreated { get; set; }
public string UserId { get; set; }
public User UserWhoCreatedRoutine { get; set; }
}
The inheritance approach was Table per Hierarchy.
I've tried to add to mapping configuration this:
modelBuilder.Entity<CustomRoutine>().HasOptional(c => c.Product)
.WithMany()
.HasForeignKey(c => c.ProductId)
.WillCascadeOnDelete(false);
But the error was same. I am not sure why this is happening, because, as you can see in the code same mapping was already done (without any problems) for RoutineLevel, also I have same mapping for Product and the other class, again with no problems.
EDIT
Here is also Product class:
public class Product
{
public Guid Id { get; protected set; }
public string Code { get; set; }
public string Name { get; set; }
public bool IsFree { get; set; }
public ICollection<SubscriptionProduct> SubscriptionProducts { get; set; }
}
And FluentAPI mapping:
modelBuilder.Entity<Product>()
.HasKey(p => p.Id);
modelBuilder.Entity<Product>()
.Property(p => p.Code)
.IsRequired()
.HasMaxLength(10);
modelBuilder.Entity<Product>()
.Property(p => p.Name)
.IsRequired()
.HasMaxLength(100);
modelBuilder.Entity<Product>()
.HasMany(p => p.SubscriptionProducts)
.WithRequired()
.HasForeignKey(p => p.ProductId)
.WillCascadeOnDelete(true);