Not able to figure out why i get the error "Unable to cast object of type Generic List to type 'NHibernate.Collection.IPersistentCollection'"

[aswinfrancis]

My parent class:
public class SomeData

{
	public virtual Schedule schedule { get; set; }
	public virtual IEnumerable<SomeUser> SomeUsers{get;set}   
}

Child class:

public class SomeUser : Entity
{
	public virtual SomeData SomeData { get; set; }
	public virtual IUser AssignedTo { get; set; }      
}

SomeData has One-To-Many relation with SomeUser. Mapping is as below:

public class SomeDataMapping : NHibernateClassMapping
{
public SomeDataMapping()
{
References(x => x.Schedule).LazyLoad();
HasMany(x => x.SomeUser ).Inverse().Cascade.All()
.KeyColumn("Schedule_Id").Table("SomeUsers");
}

public class SomeUserMapping : ClassMap<SomeUser>
{
	protected SomeUsersMapping()
	{
		Id(x => x.Id);
		References(x => x.SomeData).Column("Schedule_Id").Class<SomeData>().Not.Nullable();
		References(x => x.AssignedTo).Column("```

AssignedTo_Id").Class().Not.Nullable();
Table("SomeUsers");
}
}
}

During creation of the first Schedule which loads these data, it gets saved fine but if I try to edit and save. I get the below error:

> System.InvalidCastException: Unable to cast object of type 
> 'WhereSelectListIterator`2[System.Int64,Project.Schedules.SomeUsers]' to type 'NHibernate.Collection.IPersistentCollection'.
>    at NHibernate.Event.Default.FlushVisitor.ProcessCollection(Object collection, CollectionType type)
>    at NHibernate.Event.Default.AbstractVisitor.ProcessEntityPropertyValues(Object[] values, IType[] types)
>    at NHibernate.Event.Default.DefaultFlushEntityEventListener.OnFlushEntity(FlushEntityEvent event)
>    at Project.ORM.Core.Configurations.CourseFlushEntityEventListener.OnFlushEntity(FlushEntityEvent e) in 

   
Below code is where I map view data to domain object

schedule.SomeData.SomeUsers = viewDto.AssignedUserIds.Select(id => new SomeUser
{
SomeData = schedule.SomeData,
AssignedTo = userRepo.FindBy(id)
});

The return type here is IEnumerable. If I convert it to List() and try to save, i get the below error

> System.InvalidCastException: Unable to cast object of type 
> 'System.Collections.Generic.List`1[Project.Schedules.SomeUsers]' to type 'NHibernate.Collection.IPersistentCollection'.

I have tried setting SomeUser as IEnumberable, ICollection, IList, tried updated the mapping using different option but not able to get over this. I recently implement another HasMany relation mapping the exact same way and it works fine .