On a standard AZERTY keyboard, we have 2 keys "-".
One on the "6" key that we can access with Shift + 6 and one at the top right of the keypad.

None of the following combination works: 

• CTRL + SHIFT + 6
• CTRL + 6
• CTRL + NUMPAD -
• CTRL + SHIFT + NUMPAD -

On the other hand, the zoom in works correctly with CTRL + SHIFT + = (even if technically SHIFT + the key "=" is actually rendering a "+".

Also the reset works fine with CTRL + 0.

Cheers from France, 

Max.