This thread on Apple's Aperture discussion forum includes a fix for a bug that prevents Automator actions from working correctly under Leopard. It manifests itself with the error:

-NSNull length: unrecognized selector sent to instance 0xa063f020

Berend posts this explanation and fix:

What has happened is that the Aperture Automator actions have been removed from /System/Library/Automator on installing Leopard (probably with Archive and Install).

I've been having this same issue and couldn't for the life of me figure out what was wrong.
The hint provided by the previous poster put me on the right track.

The Aperture Automator actions are located in the the Aperture application.

Steps to take for repairing the situation:

1. Open a Finder window and navigate to /System/Library/Automator
2. Open a second Finder window and navigate to /Applications/Aperture
3. Execute a Show package Contents of Aperture (you should get a third Finder window).
4. Now navigate to Contents/Library/Automator
5. Select everything (.definition and .action) in that folder
6. Copy with option drag to the folder in the window created in step 1.
You will be asked for the administrator password.
7. Close superfluous windows.
8. Start Automator and enjoy.

In short: copy the contents of /Applications/Aperture.app/Contents/Library/Automator to
/System/Library/Automator

I've done this and no longer get the NS Null error messages. I can now access the actions and will see if they actually work some time in the next few days.

Cause: Apple Leopard Installer.
Remedy: Apple should have warned beforehand that something like this would happen and should have provided the solution.

Hopefully the expected updated to Mac OS X and Aperture will fix this.

Tags: Aperture, Bugs, Leopard, Automation