Help for prefix is undefined (C-c C-h)

[shunlog]

From the Emacs manual:

C-h or F1 mean “help” in various other contexts as well. For instance, you can type them after a prefix key to view a list of the keys that can follow the prefix key. (You can also use ? in this context. A few prefix keys don’t support C-h or ? in this way, because they define other meanings for those inputs, but they all support F1.)

However, typing , c , h doesn't work, and instead the message is displayed Devil: C-c C-h is undefined.
None of these work:

• , c , h
• , c C-h
• , c <f1>
• , x , h (so probably any prefix launched with Devil)