diff --git a/lib/net/http.rb b/lib/net/http.rb
index e65a6005251f77..c05c5baa6cde45 100644
--- a/lib/net/http.rb
+++ b/lib/net/http.rb
@@ -1615,7 +1615,7 @@ def connect
                                       write_timeout: @write_timeout,
                                       continue_timeout: @continue_timeout,
                                       debug_output: @debug_output)
-          buf = "CONNECT #{conn_address}:#{@port} HTTP/#{HTTPVersion}\r\n"
+          buf = +"CONNECT #{conn_address}:#{@port} HTTP/#{HTTPVersion}\r\n"
           buf << "Host: #{@address}:#{@port}\r\n"
           if proxy_user
             credential = ["#{proxy_user}:#{proxy_pass}"].pack('m0')