Internet sharing script

I’ve been trying to make internet sharing a little easier by automating the setup process. I’ve done pretty good so far, but cannot get Applescript to click the “Start” button in the new sheet dialog after clicking “Start” in the main window” it keeps telling me, “System Events got an error: NSReceiverEvaluationScriptError: 4.” Here’s what I’ve got so far (the trouble spot is commented out)…

tell application "System Preferences"
	set current pane to pane ""
end tell
tell application "System Events"
	get properties
	tell process "System Preferences"
		tell window "Sharing"
			tell tab group 1
				click radio button "Internet"
				if value of static text 1 is "Internet Sharing Off" then
					if value of pop up button 1 is not "Built-in Ethernet" then
						click pop up button 1
						click menu item "Built-in Ethernet" of menu 1 of pop up button 1
					end if
					tell table 1 of scroll area 1
						set selected of row 3 to true
						if value of checkbox 1 of row 3 is 0 then
							click checkbox 1 of row 3
						end if
					end tell
					click button "Start"
					(*click button "Start" of sheet 1*)
				end if
			end tell
		end tell
	end tell
end tell

Any help would be greatly appreciated.