Have you installed a program that may have 'helped' you by installing an earlier version of either the AppleScript extension or the AppleScriptlib. I found that several programs use the library, especially, and some, if they don't find the specific version in vogue at the time the program was written, help by replacing version 1.1.4 with 1.1.3 (for example).
The current versions are 1.4.3 for the extension and 1.4 for the library. Those are available in the AppleScript update at apple.com, support, software, if they are not already installed on your system. The 1.4.3 update also updates the script editor.