diff --git a/plover_stenograph_usb.py b/plover_stenograph_usb.py
index 76092c9..2788e6c 100644
--- a/plover_stenograph_usb.py
+++ b/plover_stenograph_usb.py
@@ -539,23 +539,23 @@ class Stenograph(ThreadedStenotypeBase):
         if response is None:
             # No response implies device connection issue.
             raise IOError()
-        elif response.packet_id == StenoPacket.ID_ERROR:
+        if response.packet_id == StenoPacket.ID_ERROR:
             # Writer may reply with an error packet.
             error_number = response.p1
             if error_number == 3:
                 raise UnableToPerformRequestException()
-            elif error_number == 7:
+            if error_number == 7:
                 raise FileNotAvailableException()
-            elif error_number == 8:
+            if error_number == 8:
                 raise NoRealtimeFileException()
-            elif error_number == 9:
+            if error_number == 9:
                 raise FinishedReadingClosedFileException()
-        else:
-            # Writer has returned a packet.
-            if (response.packet_id != request.packet_id
-                    or response.sequence_number != request.sequence_number):
-                raise ProtocolViolationException()
-            return response
+            raise RuntimeError('unknown response error: %u' % error_number)
+        # Writer has returned a packet.
+        if (response.packet_id != request.packet_id
+            or response.sequence_number != request.sequence_number):
+            raise ProtocolViolationException()
+        return response
 
     def run(self):