invalid url - my config or invalid request?

[Daniel Miller]

My site is generally doing exactly what I want.  Periodically I'll see 
some errors in the log.  I'm trying to determine if these indicate 
problems in my config, or potential attacks, or simply a broken client.

The last few lines in my log:
2016/10/05 14:38:37 [error] 17912#0: *17824 invalid url, client: 
195.154.181.113, server: amfes.com, request: "HEAD /robots.txt HTTP/1.0"
2016/10/05 19:47:27 [error] 17912#0: *18315 invalid url, client: 
169.56.71.56, server: amfes.com, request: "GET / HTTP/1.0"
2016/10/08 13:46:21 [error] 17910#0: *27413 invalid url, client: 
212.83.162.138, server: amfes.com, request: "HEAD /robots.txt HTTP/1.0"
2016/10/09 18:05:30 [error] 17912#0: *32588 invalid url, client: 
211.1.156.90, server: amfes.com, request: "HEAD / HTTP/1.0"

Clients I control have no problem reaching the root or the robots.txt 
file - so what is this telling me?

-- 
Daniel