Keybinding ctrl-shift-<letter> doesn't work

The documentation indicates it should be possible to keybind ctrl+<uppercase letter>, but in practice I haven't been able to get it to work.

Traceback (most recent call last):
  File "app.py", line 10, in <module>
    from app.keybindings import keybindings
  File "/Users/user/code/cli-app/app/keybindings.py", line 43, in <module>
    @keybindings.add('c-Y')
  File "/Users/user/code/cli-app/env/lib/python3.7/site-packages/prompt_toolkit/key_binding/key_bindings.py", line 188, in add
    keys = tuple(_check_and_expand_key(k) for k in keys)
  File "/Users/user/code/cli-app/env/lib/python3.7/site-packages/prompt_toolkit/key_binding/key_bindings.py", line 188, in <genexpr>
    keys = tuple(_check_and_expand_key(k) for k in keys)
  File "/Users/user/code/cli-app/env/lib/python3.7/site-packages/prompt_toolkit/key_binding/key_bindings.py", line 337, in _check_and_expand_key
    raise ValueError('Invalid key: %s' % (key, ))
ValueError: Invalid key: c-Y

It looks like those key combos would need to be added to python-prompt-toolkit/prompt_toolkit/keys.py.
I'm not sure if this is a bug, or if I'm just misunderstanding how this works (I've just started learning prompt-toolkit).

Start by reading the special-key documentation and inspecting python-prompt-toolkit/prompt_toolkit/keys.py, then trace how key_bindings.py validates the c-Y key name. Confirm the intended uppercase control-key syntax and add the necessary handling so the documented binding is accepted, with coverage for the reported case.