diff --git a/test/lib/test/unit/parallel.rb b/test/lib/test/unit/parallel.rb
index 6a319c3c3f..1e12d18457 100644
--- a/test/lib/test/unit/parallel.rb
+++ b/test/lib/test/unit/parallel.rb
@@ -139,7 +139,7 @@ module Test
         rescue Errno::EPIPE
         rescue Exception => e
           begin
-            trace = e.backtrace
+            trace = e.backtrace || ['unknown method']
             err = ["#{trace.shift}: #{e.message} (#{e.class})"] + trace.map{|t| t.prepend("\t") }
 
             _report "bye", Marshal.dump(err.join("\n"))