How to avoid InvalidOperationException when setting the defaultbutton in ASPX content

I am trying to set a default button in my ASPX page. I have a master page so the form is there. I have a panel in the content page that holds a table that organizes a number of textboxes, dropdowns and other inputs. The last row of the table holds some buttons, one of which I want to be the default button. After doing some research, I have tried the following with no success.

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    If Not IsPostBack Then
       pnlHolder.DefaultButton = cmdSearchJob.ClientID

I have also tried

pnlHolder.DefaultButton = cmdSearchJob.UniqueID

and

Dim cmdDef As Button = pnlHolder.FindControl("cmdSearchJob")
pnlHolder.DefaultButton = cmdDef.UniqueID

but both throw the exception "The DefaultButton of 'pnlHolder' must be the ID of a control of type IButtonControl.".

I have seen some Javascript solutions, but was hoping to just be able to set the defaultButton for the panel.

Finally found what it was. Tried adding another button with no CSS, etc. that just popped up a javacsript alert() for testing. Was able to set that as the default button when it was outside the table. As the submit button is one of a series of buttons (not a very pretty UI, but user requirements and all that) in the table, I used this additional button and set its style to display:none and have it call the same subroutine in the code behind.

So, short answer, it wasn't seeing it in the table.

Thanks everyone for your input.

I still have no idea why it wouldn't set the button.