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.

What is causing this and how do I fix it?

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