Populate GridView Cell value using ListBox in ASP.Net

Trouble to retriving gridview cell value in Listbox.

Code:

 textbox1.Text=grddisplay.Rows[grddisplay.SelectedIndex].Cells[14].Text;
 Listbox1.Text=grddisplay.Rows[grddisplay.SelectedIndex].Cells[14].Text;

textbox code work perfectly

Problem in listbox how to solve