To resolve this run the same operation through a query analyser session. You can get the script for the operation you are performing by first:

Carry out the operation in enterprise manager.

Before saving your changes, click the 'Generate change script' icon in the top left of enterprise manager.

This will then give you a script version of your change.

Copy this into query analyser

Do not save your changes from the gui alterations

Run the script from query analyser

Your action will complete correctly.