Mapping + as shorcut does not work as expected

[Patrik Svestka]

I want to use + as a shorcut for adding a node into a tree.

I'm using the following code to add + as a shortcut

    | keyboardMapping |

    keyboardMapping := Display keyboardMap.
    keyboardMapping bindValue: #Add to: #+

This does not work properly even when I check the mappings via Display keyboardMap it shows up at #bindings (first from top).

On the other hand, when I do different mappings like:(replacing the last line):
keyboardMapping bindValue: #Add to: #Ctrld

When I press Ctrl+d it works as expected:

Same goes to #- as alias.

Why can't #+ (#-) be used as a shortcut?