arrays - Object reference not set to an instance of an object

The purpose of this program is to run through a range of pictureboxes and set it's .image property to a particular image. I keep getting the error "Object reference not set to an instance of an object". Coming from the line that shows "DirectCast(Me.Controls(pic(i)), PictureBox).Image = My.Resources.glass_buttonred"....Whats weird is if i move that code outside of the for loop it runs just fine.

Private Sub Button1_Click(sender As System.Object, e As System.EventArgs) Handles Button1.Click
    Dim pic(2) As Object
    For i = 0 To 2
        pic(i) = "picturebox" + Convert.ToString(i)
        DirectCast(Me.Controls(pic(i)), PictureBox).Image = My.Resources.glass_buttonred
    Next
    Label1.Text = pic(1)
End Sub

HERE IS THE WORKING CODE. THANKS! Hope it will help others wanting to convert string to control object

Dim pic(2) As Object
For i = 0 To 2
    pic(i) = "picturebox" + Convert.ToString(i + 1)
    DirectCast(Me.Controls(pic(i)), PictureBox).Image = My.Resources.glass_buttonred
Next
Label1.Text = pic(1)