"An error has occured in approval of changes"

A business user setup the OOB Approval workflow for a document library. The Approval workflow completed fine. However, at the end of the workflow, it gave the "An error has occured in approval of changes" error! After a little bit of playing around, I figured that Content Approval for the document library needs to be enabled in order for the Approval workflow to complete successfully.

Steps to enable Content Approval are:
1. Go to Document Library settings.
2. Click on Versioning Settings under General Settings
3. Set "Require content approval for submitted items?" to Yes