Really weird request to my LOCAL development server

So basically I'm running a simple local development server (started with the command python3 -m http.server), only serving one single index.html file. It is a compiled Elm app, if it matters.

Whenever a client makes a request, I see in the terminal a message of the following sort:
127.0.0.1 - - [16/Aug/2019 19:17:27] "GET / HTTP/1.1" 304 -

I'm expecting I'd be the only one making requests, but then I see this: 51.38.36.213 - - [16/Aug/2019 18:53:00] code 400, message Bad HTTP/0.9 request type ('\x00\x00\x00') 51.38.36.213 - - [16/Aug/2019 18:53:00] " c" 400 -

What does it mean? How can I be seeing this message if it's on localhost? Should I be concerned?