According to Suetonius, the final words of emperor Vespasian were "Vae! Puto deus fio" which translates to "Alas! I think I'm becoming a god".

But the 'proper' way of saying this would be using an acc-inf construction, i.e. "Vae! Puto me deum fieri". Why did he say it the other way? I've never seen this construction before, and I've always been told the acc-inf is the right way of doing it. Is this a later Latin construction which became acceptable to use?