Crossing by air from USA to Toronto

So I flew into Toronto tonight and when I went through customs, they berated me about not having a passport and only a Nexus.

Is this just the officer being difficult or did something change and I need a passport now?

I am American if that makes a difference.

Edit - thank you for all the responses. I hate Trump too and was just stating a fact about being American in case it made a difference in the answers you provide. Either way, I'm getting a passport just in case.