"Clear" button has no effect in Settings > Window Manager > Keyboard tab

I wanted to disable some pesky keybindings which I never use except by mistake. I found that many (but not all) of the default
keybindings cannot be disabled by clicking the Clear button.

It does work as expected if I add a keybinding to a previously unbound window manager action and then press Clear to remove it again.

If you click the Edit button, it waits for you to press a new key combination to bind the action to. This is a separate, smaller dialog window which also has a Clear button. This button doesn't work, either.

Steps to repro:
- Open the Window Manager prefs: xfwm4-settings
- Select the Keyboard tab (alt-K)
- Highlight "Workspace 1" which is bound to "Ctrl+F1" by default
- Hit the Clear button

Expected outcome:
- The Ctrl+F1 binding should be cleared

Actual outcome:
- It's still there

Workaround:
- Rebind to another key: Click Edit and hit an obscure combo like Shift+Ctrl+Alt+dead circumflex
- Then clear this new keybinding by clicking Clear

I don't see any general pattern to which defaults cannot be cleared. A general observation is that the ones towards the end of the list more often seem to exhibit this problem, but it's not all of them.

$ apt-cache policy xfwm4
xfwm4:
  Installed: 4.11.1-2ubuntu2
  Candidate: 4.11.1-2ubuntu2
  Version table:
 *** 4.11.1-2ubuntu2 0
        500 http://fi.archive.ubuntu.com/ubuntu/ trusty/universe amd64 Packages
        100 /var/lib/dpkg/status

$ lsb_release -rd
Description: Ubuntu 14.04.1 LTS
Release: 14.04