Slavery and property rights

Having just come across The Institute for New Economic Thinking Blog I started reading some of the posts there and am more than just a bit puzzled by the following comment

Strong enforcement of property rights is good for economic growth, says the conventional wisdom. The link may not be as clear cut, says Suresh Naidu. He and co-investigator Jeremiah Dittmar are digging through court records and newspaper ads on runaway slaves to come up with a measure of property rights enforcement. The hypothesis is that weak enforcement of property rights in people – slavery that is – discouraged investment in slaves and encouraged investment in manufacturing and infrastructure instead. A new angle on the link between property rights and economic growth – this is new economic thinking.

But if you had strong enforcement of property rights you wouldn't have slavery in the first place. Slavery is after all a rather obvious violation of property rights: the property right that says we own ourselves. Slavery is not, by definition, a voluntary act, ownership in you is taken from you by force, so I can't follow the "logic" of the quote above.