can't assign numpad keys correctly to shortcuts

[[mrs_sheep]]

uname -a : Linux XXXXXXXX-Ubuntu-x64 3.0.0-14-generic #23-Ubuntu SMP Mon Nov 21 20:28:43 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

running Unity.

I switch workspaces a lot so i found it usefull to be able to switch them even quicker by assigning the directions to the arrow keys on the numpad. My numpad is (while Numlock is off) a bunch of PageUp, PageDown, Home, End et cetera keys.
When turning numlock on, all these functions are disabled in favor to the numbers.

The problem is, that the shortcut is apparently not assigned to the Arrow but the Key itself, so pressing Num4 also triggers the shortcut to switch to the left, even while Numlock is activated. Thus, assigning shortcuts to the NumPad - Arrows makes it absolutely unusable to enter numbers, which in some way is the main use of the numpad.

What I'd like it to be like:
While the NumLock is off, i can trigger the Shortcut.
While the NumLock is on, i can type numbers.