“_split_longopt: command not found” when trying tab-completion

For the past couple of weeks or so, I’ve noticed that, often, when I press the Tab key to try and complete a command in a bash shell in a terminal, I get an error message. It says:

_split_longopt: command not found

What could cause this, and how can I fix it?

Note: I’m using Devuan Excalibur (~= Debian Trixie without systemd).