What does the error 'Exception: Failure "lexing: empty token".' mean in OCamlex?

What does the error:

Exception: Failure "lexing: empty token".

mean in OCamlex?

I’ve never seen this error, but it stands to reason that the token matched by a rule in ocamllex must be non-empty (not the empty string). Otherwise the next call will return the same empty token, and so on forever.