Illustrator Scripting :: Converting PDF To PNG?

I wrote the following AppleScript to open a PDF file and export is as PNG. I am using Illustrator CS5.
 
The export command gets an error: Adobe Illustrator got an error: Can't get current document.
when it encounters the export command.
 
set inputFile to (choose file with prompt "Open PDF file to convert to PNG:") as text
set exportFile to inputFile & ".png"
tell application "Adobe Illustrator"
if not frontmost then activate
open inputFile as alias without dialogs
export current document to exportFile as PNG24 ¬
with options {class:PNG24 export options ¬
, resolution:300 ¬
, matte color:{red:0, green:0, blue:0}}
close current document saving no
end tell