FK conflict occurred

This error is coming

The INSERT statement conflicted with the FOREIGN KEY constraint "FK_SubDepartment_SubDepartment". The conflict occurred in database "AttendanceManagement", table "dbo.Department", column

protected void Page_Load(object sender, EventArgs e)
{
{
con = new SqlConnection("Data Source=IT;Integrated Security=SSPI;Initial Catalog=AttendanceManagement");
using (SqlCommand cmd = new SqlCommand("SELECT DeptID, DeptName FROM Department"))
{
cmd.CommandType = CommandType.Text;
cmd.Connection = con;
con.Open();
DropDownList1.DataSource = cmd.ExecuteReader();
DropDownList1.DataTextField = "DeptName";
DropDownList1.DataValueField = "DeptID";
DropDownList1.DataBind();
con.Close();
}
}
DropDownList1.Items.Insert(0, new ListItem("--Select Customer--", "0"));
}
protected void Button1_Click(object sender, EventArgs e)
{
string DeptName = txtsubdept.Text;
string DeptID = DropDownList1.SelectedValue;
string query = "INSERT INTO SubDepartment VALUES(@DeptName,@DeptID)";
con = new SqlConnection("Data Source=IT;Integrated Security=SSPI;Initial Catalog=AttendanceManagement");
{
using (SqlCommand cmd = new SqlCommand(query))
{
cmd.Parameters.AddWithValue("@DeptName", DeptName);
cmd.Parameters.AddWithValue("@DeptID", DeptID);
cmd.Connection = con;
con.Open();
cmd.ExecuteNonQuery();
con.Close();