present tense mixed condition in Erasmus?

I’m (more or less) used to past mixed conditions with the protasis in the subjunctive and the apodasis in the indicative (e.g. Seneca "Mors ad te venit; timenda erat si tecum esse posset), but I’ve just come across one in the present in a letter of Erasmus and I’m having trouble remembering coming across this before:

Nam bene vīvere, nisī vīvāmus, nōn possumus.

Is this a frequent thing that I’ve just somehow failed to notice before? Or is this kind of mixed condition in the present unusual?

I wouldn’t think it’s particularly unusual. “We can’t live well if we’re not to live.” A little more subtle than the indicative perhaps. Does this fit the context? There’s a Senecan dictum on old age that I (ipse senex) take to heart: est plena voluptatis si illa scias uti. “Old age is full of pleasure if only you’d know how to use it.”

Grātum! I hadn’t run across that Seneca before—this seems to be just a construction I somehow managed either not to have encountered or (far more likely) to have encountered but not registered until now.