How do I insert record on table with one to one relationship using dapper?

So far I have inserted and retrieve records in a single table with no problem. My current situation is, I have two tables which has a one to one relationships. I have the following table structure and code, when I insert the record in the database I have this error: System.NullReferenceException: 'Object reference not set to an instance of an object.'I am all ears right now in hearing your suggestions and recommendations in moving forward since I am only new to dapper. Thank you in advance.

I have this table structure.

----------
Employee
----------
Id
LastName
FirstName
MiddleName
DivisionId

-----------
Division
-----------
Id
DivisionName


Class:

public class Employee
{
   public int Id {get;set;}
   public string LastName {get;set;}
   public string FirstName {get;set;}
   public string MiddleName {get;set;}
   public Division Division {get;set;}
}

public class Divison
{
   public int Id {get;set;}
   public string DivisionName {get;set;}
}


FORM:

UserModel u = new UserModel();

int departmentId = 0;
Int32.TryParse(divisionComboBox.SelectedValue.ToString(), out divisionId);

  
u.LastName = lastNameText.Text.ToUpper();
u.FirstName = firstNameText.Text.ToUpper();
u.MiddleName = middleNameText.Text.ToUpper();
u.Division.Id = divisionId;

DATA ACCESS:

public static void SaveUser(UserModel user)
   {
        using (IDbConnection cnn = new SqlCeConnection(LoadConnectionString()))
     {
cnn.Execute("insert into [User] ([LastName], [FirstName], [MiddleName], [DivisionId]) 
             values (@LastName, @FirstName, @MiddleName, @DivisionId)", user);
        }
    }

So far I have inserted and retrieve records in a single table with no problem.