diff --git a/lib/net/protocol.rb b/lib/net/protocol.rb
index e1586923c80028..380d31ed5d9aa7 100644
--- a/lib/net/protocol.rb
+++ b/lib/net/protocol.rb
@@ -172,8 +172,10 @@ def readline
     BUFSIZE = 1024 * 16
 
     def rbuf_fill
-      case rv = @io.read_nonblock(BUFSIZE, exception: false)
+      tmp = @rbuf.empty? ? @rbuf : nil
+      case rv = @io.read_nonblock(BUFSIZE, tmp, exception: false)
       when String
+        return if rv.equal?(tmp)
         @rbuf << rv
         rv.clear
         return