Drag and Drop on a TableView, NSDraggingInfo error

Hey guys, I'm trying to implement drag and drop reordering on a tableView, but when I try to implement this method in my delegate:

Code:

- (NSDragOperation)tableView:(NSTableView*)tv validateDrop:(id <nsdraggingInfo>)info proposedRow:(int)row proposedDropOperation:(NSTableViewDropOperation)operation

and build, I get this error:

Every source I've looked at uses that method to implement drag and drop reordering, but no one has ever talked about that error, so I've no clue what's going on here. Anyone have any ideas?