Warning: Computing just-done stamp in plan NIL for action

More new warnings from ASDF 3.3, this time I have no idea what it means. WARNING: Computing just-done stamp in plan NIL for action (ASDF/LISP-ACTION:LOAD-OP "system"), but dependency (ASDF/FIND-SYSTEM:DEFINE-OP "system") wasn't done yet! The .asd file is not public, but it looks perfectly normal without using any complicated features.