diff --git a/lib/net/protocol.rb b/lib/net/protocol.rb
index 197ea09..3933bfe 100644
--- a/lib/net/protocol.rb
+++ b/lib/net/protocol.rb
@@ -195,7 +195,10 @@ def readuntil(terminator, ignore_eof = false)
       offset = @rbuf_offset
       begin
         until idx = @rbuf.index(terminator, offset)
-          offset = @rbuf.bytesize
+          new_offset = @rbuf.bytesize - terminator.bytesize + 1
+          # Only assign the offset if it will advance.
+          # Otherwise an empty @rbuf could result in a negative offset.
+          offset = new_offset if new_offset > offset
           rbuf_fill
         end
         return rbuf_consume(idx + terminator.bytesize - @rbuf_offset)
diff --git a/test/net/protocol/test_protocol.rb b/test/net/protocol/test_protocol.rb
index 2f42fa3..bc88bf7 100644
--- a/test/net/protocol/test_protocol.rb
+++ b/test/net/protocol/test_protocol.rb
@@ -27,6 +27,26 @@ def test_each_crlf_line
     end
   end
 
+  def test_each_message_chunk_boundary
+    assert_output("", "") do
+      # Create this first chunk where the chunk will end with the
+      # line terminator \r\n only being partially read up to and including \r.
+      chunk_1 = "#{"1" * (Net::BufferedIO::BUFSIZE - 1)}\r\n".b
+      chunk_2 = "Second line\r\n".b
+      chunk_3 = "Third line\r\n".b
+      test_string = "#{chunk_1}#{chunk_2}#{chunk_3}".b
+      sio = StringIO.new("#{test_string}.\r\n".b)
+      io = Net::InternetMessageIO.new(sio)
+      expected_chunks = []
+      io.each_message_chunk do |chunk|
+        expected_chunks << chunk
+      end
+      assert_equal chunk_1, expected_chunks[0]
+      assert_equal chunk_2, expected_chunks[1]
+      assert_equal chunk_3, expected_chunks[2]
+    end
+  end
+
   def create_mockio(capacity: 100, max: nil)
     mockio = Object.new
     mockio.instance_variable_set(:@str, +'')