One - one relationship prevents the primary table from inserting data!

I have those two entities, 

public class PatientRegistry {
        [DatabaseGenerated (DatabaseGeneratedOption.Identity)]
        [Display (Name = "Record Id")]
        public long RecordId { get; set; }

        [Key, DatabaseGenerated (DatabaseGeneratedOption.None)]
        [Display (Name = "Patient File Number")]
        public long PatientFileId { get; set; }

        [Required, StringLength (50)]
        [Display (Name = "First Name")]
        public string FirstName { get; set; }
public virtual ICollection<PartnerRegistry> Partners { get; set; }
        // public virtual ICollection<FileUpload> FileUpload { get; set; }
        public virtual PatientAccount PatientAccount { get; set; }
}

and 

public class PatientAccount {
        [Key, DatabaseGenerated (DatabaseGeneratedOption.Identity)]
        public long RecordId { get; set; }
        [Required, StringLength (15)]
        public string MobileNo { get; set; }
        public bool IsConfirmedPhoneNumber { get; set; } = false;
        public bool IsConfirmedEmailAddress { get; set; } = false;
        public bool IsLocked { get; set; } = false;
        public int? FailedAttempts { get; set; }
        public DateTimeOffset? LastLoggedIn { get; set; }
        public DateTimeOffset DateCreated { get; set; }
        [Timestamp]
        public byte[] RowVersion { get; set; }
        public long PatientFileId { get; set; }
        public virtual PatientRegistry PatientFile { get; set; }
    }

and I am defining the relationship using the fluent API, 

builder.Entity<PatientRegistry> ()
                                .HasOne (a => a.PatientAccount)
                                .WithOne (b => b.PatientFile)
                                .HasForeignKey<PatientAccount> (c => c.PatientFileId)
                                .OnDelete (DeleteBehavior.Cascade);

now I am trying to seed into the `PatientRegistry` table, I keep getting an error about null data in the PatientAccount table!!!!, why is this happening?