diff --git a/lib/websocket/driver/hybi.rb b/lib/websocket/driver/hybi.rb
index 3487c4a..e823c48 100644
--- a/lib/websocket/driver/hybi.rb
+++ b/lib/websocket/driver/hybi.rb
@@ -322,6 +322,10 @@ def parse_length(octet)
         if @require_masking and not @frame.masked
           return fail(:unacceptable, 'Received unmasked frame but masking is required')
         end
+
+        if @masking and @frame.masked
+          return fail(:protocol_error, 'Received masked frame but masking is forbidden')
+        end
       end
 
       def parse_extended_length(buffer)