ProDOS: Error 900 When Using Apple IIc Sys Utilities Disk

I started up my system with the Apple IIc System Utilities Disk. This disk has a new port configuration saved to it. If I select any menu option then press the Escape key to return to the main menu, I always drop into the monitor with Error #900 displayed. How can I get around this?
The workaround is as follows:

1. Duplicate your Systems Utilities disk: select the main menu option 5. Boot your system with this new copy.

2. Select main menu option 9 to exit from System Utilities.

3. From the Basic prompt, type in the following:

Load SU,S6
List 900

You will see the listing: 900 Call 768: Poke 216,0: Onerr Goto 930

4. Change line 900 to: 900 Poke 216,0: Onerr Goto 930: Call 768

5. Enter the following commands to unlock the file, save the changes to disk and relock the file to prevent accidental damage: List 900

6. Check once again that line 900 reads properly as in step 4, then type:

Unlock SU,S6
Save SU,S6
Lock SU,S6
Published Date: Feb 18, 2012