Help: 9yo lying about homework situation

So my oldest has been saying his math homework is finished when it clearly isn't. I found incomplete assignments in his backpack. The teacher emailed me about it too. I'm trying not to overreact but I'm also not sure what the right move is here. Do I just let natural consequences happen (bad grade)? Ground him? Have a calm conversation about honesty?

I grew up with a dad who yelled about stuff like this and I really don't want to do that. But I also can't just ignore it. Advice?