Citoyenne la traductrice jacobine

M. Cottret claims (Tuer le tyran ? p. 344) that Robespierre was out sick on the day of Saint-Just's first speech calling for Louis Capet's execution, but she doesn't give a source. (It's possible that it also comes from Vinot, since she cites him a few lines previously, but it's hard to tell.) Does anyone know what the source might be?

Because if it's true - and I doubt M. Cottret would say so if she at least didn't have good reason to believe it - then that rather puts a damper on all those awful, cliché scenes in fiction where Robespierre is either putting him up to it or suddenly inspired to turn into an evil fanatic while hearing the speech. And anything that would prove the logical impossibility of such scenes, would make my evening.