'Super' key cannot be assigned as a modifer in settings dialogs

While the Super key works fine as a shortcut modifier if it's already been configured as one, if you try to use xfwm4-settings (or xfdesktop-settings) to configure a shortcut, Super doesn't work.

The dialog that pops up to interactively show the keys pressed does correctly show Super, but when pressing a non-modifier key to lock in the shortcut, the result just shows non-Super modifiers and the non-modifier key.