Comparative construction

Hello, everyone!

I could, once again, use some help. I'm working on comparative construction in my workbook at the moment, and have been stumped. Yes, it is a pretty simple topic, but there is a grammatical difference between the different sources I am using, causing me to be confused.

The given sentence is pretty elementary-level:
"His work is (much) better than mine."

The book which teaches the material says:
"Его работа (намного) лучше, чем моя."

The workbook says the correct answer is:
"Его работа (намного) лучше, чем моей."

Which grammatical form is correct? Genitive or nominative?

Thank you!