PyMongo upsert throws "upsert must be an instance of bool" error

I'm running an update on my MongoDB from Python. I have this line:

self.word_counts[source].update({'date':posttime},{"$inc" : words},{'upsert':True})

But it throws this error:

raise TypeError("upsert must be an instance of bool")

But True looks like an instance of bool to me!

How should I correctly write this update?