diff --git a/ChangeLog b/ChangeLog
index 7c086441cf..d6d37498c8 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+Wed Nov 25 19:30:30 2009  Martin Duerst  <duerst@it.aoyama.ac.jp>
+
+	* transcode.c: Added a check for an internal error
+	  (with Tatsuya Mizuno)
+
 Tue Nov 24 22:57:46 2009  Tanaka Akira  <akr@fsij.org>
 
 	* test/ruby/envutil.rb (EnvUtil.invoke_ruby): raise Timeout::Error
diff --git a/transcode.c b/transcode.c
index 22b44d8280..01f3245742 100644
--- a/transcode.c
+++ b/transcode.c
@@ -674,6 +674,8 @@ transcode_restartable0(const unsigned char **in_pos, unsigned char **out_pos,
             goto invalid;
 	  case UNDEF:
 	    goto undef;
+	  default:
+	  	rb_raise(rb_eRuntimeError, "unknown transcoding instruction");
 	}
 	continue;