MdiParent Form1 than button Form2 open

System.ArgumentException: 'Form that was specified to be the MdiParent for this form is not an MdiContainer.

Parameter name: value'

private void btnshow_Click(object sender, EventArgs e)
 {
        Crysatal_Report_Charges_Type cp = new Crysatal_Report_Charges_Type();
        cp.MdiParent = this;
        cp.Show();
 }