The Reason Phrase on a response is optional.

The standard has it as *<TEXT, exlcuding CR, LF>
and a response with no reason phrase was found in
the wild.

http_parser.rl

@@ -378,8 +378,8 @@ do {                                                                 \
   Request_Line = ( Method " " Request_URI ("#" Fragment)? " " HTTP_Version CRLF ) ;
 
   StatusCode = (digit digit digit) $status_code;
-  ReasonPhrase =  ascii+ -- ("\r" | "\n");
-  StatusLine = HTTP_Version  " " StatusCode " " ReasonPhrase CRLF;
+  ReasonPhrase =  ascii* -- ("\r" | "\n");
+  StatusLine = HTTP_Version  " " StatusCode (" " ReasonPhrase)? CRLF;
 
 # chunked message
   trailing_headers = header*;

test.c

@@ -354,6 +354,16 @@ const struct message responses[] =
   ,.body= ""
   }
 
+, {.name= "301 no response phrase"
+  ,.type= HTTP_RESPONSE
+  ,.raw= "HTTP/1.1 301\r\n\r\n"
+  ,.should_keep_alive = TRUE
+  ,.status_code= 301
+  ,.num_headers= 0
+  ,.headers= {}
+  ,.body= ""
+}
+
 , {.name= NULL } /* sentinel */
 };