C sharp - NO muestra check box en gridview

DataGridViewCheckBoxColumn checkboxColumn = new DataGridViewCheckBoxColumn();

checkboxColumn.Width = 30;

checkboxColumn.Name = "chk";

checkboxColumn.HeaderText = "Sel.Celular";

checkboxColumn.ReadOnly = false;

checkboxColumn.DefaultCellStyle.Alignment = DataGridViewContentAlignment.MiddleCenter;

this.dataGridViewAgenda.Columns.Insert(18, checkboxColumn);