Document.getPreviousDocument() and Document.getPreviousVersion() seems not to work

Reproductibility :

• go to any page with at least two versions in the history, edit it, write :
• $doc.getPreviousVersion()
• $doc.getPreviousDocument().getVersion()

result : doc.getPreviousVersion() returns "" and doc.getPreviousDocument().getVersion() returns the first version of the document.