MySQL hiba

Hiba az alábbi parancs végrehajtása közben:
SELECT count(*) AS dbszam FROM muvek m JOIN tagek2muvek t JOIN tagek k JOIN users u WHERE k.url_cim = "aktualis-palyazatok" AND m.mu_statusza = 100 AND m.mu_id = t.mu_id AND k.tag_id = t.tag_id AND m.szerzo_user_id = u.id AND m.release_date <= NOW() ORDER BY release_date DESC,upload_date DESC

1052: Column 'upload_date' in order clause is ambiguous

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