diff --git a/test/lib/test/unit.rb b/test/lib/test/unit.rb
index 2cd554bd66..d886f29c73 100644
--- a/test/lib/test/unit.rb
+++ b/test/lib/test/unit.rb
@@ -325,7 +325,7 @@ module Test
       def deal(io, type, result, rep, shutting_down = false)
         worker = @workers_hash[io]
         cmd = worker.read
-        cmd.sub!(/\A\.+/, '')
+        cmd.sub!(/\A\.+/, '') if cmd # read may return nil
         case cmd
         when ''
           # just only dots, ignore