src/Pure/System/system_channel.ML

     val (in_stream, out_stream) = Socket_IO.open_streams name;
     val _ = BinIO.StreamIO.setBufferMode (BinIO.getOutstream out_stream, IO.BLOCK_BUF);
   in
     System_Channel
      {input_line = fn () => read_line in_stream,
-      inputN = fn n => Byte.bytesToString (BinIO.inputN (in_stream, n)),
+      inputN = fn n => if n = 0 then "" else Byte.bytesToString (BinIO.inputN (in_stream, n)),
       output = fn s => BinIO.output (out_stream, Byte.stringToBytes s),
       flush = fn () => BinIO.flushOut out_stream}
   end;
 
 end;