DbUpdateException was unhandled by user code error

Question

The error code given was quite vague so I'm having a hard time tracing the error. I'm just assuming that it has something to do with how I related my table?

The error code is this:

{"The INSERT statement conflicted with the FOREIGN KEY constraint \"FK_dbo.LGMembers_dbo.Lifegroups_LifegroupID\". 
The conflict occurred in database \"aspnet-COGSimple-20140204091131\", table \"dbo.Lifegroups\", column 'LifegroupID'.\r\nThe statement has been terminated."}

From this post INSERT statement conflicted with the FOREIGN KEY constraint, It said that:

The way a FK works is it cannot have a value in that column that is not also in the primary key column of the referenced table

However, my tables are referenced in such a way that every fk is linked to a Primary Key.

The tables affected:

public class Lifegroup
{
    [Key]
    public int LifegroupID { get; set; }
    [Display(Name = "Lifegroup Name")]
    public string LGName { get; set; }
    [Display(Name = "Lifegroup Type")]
    public string LGType { get; set; }
    public DateTime DateCreated { get; set; }

    public int LGLeaderID { get; set; }

    [ForeignKey("LGLeaderID")]
    public virtual LGLeader LGLeader { get; set; }
}

public class LGLeader
{
    [Key]
    public int LGLeaderID { get; set; }

    public string UserID { get; set; }

    [ForeignKey("UserID")]
    public virtual User User { get; set; }
}

public class LGMember
{
    [Key]
    public int LGMemberID { get; set; }

    public string UserID { get; set; }
    public int LifegroupID { get; set; }

    [ForeignKey("UserID")]
    public virtual User User { get; set; }
    [ForeignKey("LifegroupID")]
    public virtual Lifegroup Lifegroup { get; set; }
}

Thank you very much!

Edit:

The error comes from a db.SaveChanges() when trying to insert data to LGMember.