Meaning of "Session object destruction failed" error?

Well, this question has been asked many times over the years in the archives, but I didn't see any answer. Does anyone know the cause of this error: "Session object destruction failed"? The error is not caused by not starting a session, or, the session timing out, before calling session_destroy(); the error message in those cases is "Trying to destroy uninitialized session". Here is the PHP source code where the message is generated. if (PS(mod)->destroy(&PS(mod_data), PS(id)) == FAILURE) { retval = FAILURE; php_error(E_WARNING, "Session object destruction failed"); } Any help appreciated! Kirk