I've read that muCommander has an internal binary editor, but I just can't manage to launch it.

In many cases, when I select a binary file (for example a PDF file) and press F4, I get the following error:

In other cases, muCommander opens the file in its internal editor in text mode, but when I then go to the menu item Editor > Binary, nothing happens.

By the way, the internal binary viewer does work fine.

Here's my OS version and muCommander version: