qgis - Labels cannot be moved although data-defined settings are active

I'm trying to manually move labels but having some trouble. I'm trying to move the labels while in edit mode. I've added fields for x coordinate (all null), y coordinate (all null), rotation (all =0), and font-size (all =10). This makes the "move label" and "rotate label" buttons active, but when I click on the labels, no box around the label appears and it's impossible to move the label. What am I doing wrong?