Display dialog with icon filename

Hi, I'm trying to use a display dialog with an icon specified by filename.

The following specimen code works if I put the display dialog inside the first tell statement (before the second tell statement), but fails (with the error shown at the bottom) if the display dialog is within the second tell statement.

tell application "Numbers"

activate

open (a .numbers file)

tell front document

display dialog "A message to the world." buttons {"OK"} with icon file "MacIntoshHD:Users:pathto.icns"

end tell

end tell

THE ERROR PRODUCED BY THE ABOVE CODE

Numbers got an error: Can’t make file "MacIntoshHD:Users:pathto.icns" of document 1 into type number or string.

I'm obviously missing something, but I can't see what. Any ideas?

Thanks.

tell a