diff --git a/lib/net/protocol.rb b/lib/net/protocol.rb
index f241681..aba6557 100644
--- a/lib/net/protocol.rb
+++ b/lib/net/protocol.rb
@@ -120,6 +120,7 @@ def initialize(io, read_timeout: 60, write_timeout: 60, continue_timeout: nil, d
       @continue_timeout = continue_timeout
       @debug_output = debug_output
       @rbuf = ''.b
+      @rbuf_empty = true
       @rbuf_offset = 0
     end
 
diff --git a/test/net/protocol/test_protocol.rb b/test/net/protocol/test_protocol.rb
index d3dc2cc..397cc72 100644
--- a/test/net/protocol/test_protocol.rb
+++ b/test/net/protocol/test_protocol.rb
@@ -57,6 +57,14 @@ def mockio.write_nonblock(*strs, exception: true)
     mockio
   end
 
+  def test_readuntil
+    assert_output("", "") do
+      sio = StringIO.new("12345".dup)
+      io = Net::BufferedIO.new(sio)
+      assert_equal "12345", io.readuntil("5")
+    end
+  end
+
   def test_write0_multibyte
     mockio = create_mockio(max: 1)
     io = Net::BufferedIO.new(mockio)