diff --git a/database.cc b/database.cc
index b7fb1f10..f01ffa4b 100644
--- a/database.cc
+++ b/database.cc
@@ -129,6 +129,7 @@ _internal_error (const char *format, ...)
 
     va_start (va_args, format);
 
+    fprintf (stderr, "Internal error: ");
     vfprintf (stderr, format, va_args);
 
     exit (1);
diff --git a/message.cc b/message.cc
index 60ddf8a8..75e752c8 100644
--- a/message.cc
+++ b/message.cc
@@ -276,8 +276,10 @@ notmuch_message_get_thread_id (notmuch_message_t *message)
 	strncmp ((*i).c_str (), _find_prefix ("thread"),
 		 strlen (_find_prefix ("thread"))) == 0)
     {
-	INTERNAL_ERROR ("Message with document ID of %d has duplicate thread IDs.\n",
-			message->doc_id);
+	INTERNAL_ERROR ("Message %s has duplicate thread IDs: %s and %s\n",
+			notmuch_message_get_message_id (message),
+			message->thread_id,
+			(*i).c_str () + 1);
     }
 #endif