diff --git a/include/tts/TtsEngine.h b/include/tts/TtsEngine.h
index 06f3820..bf62995 100644
--- a/include/tts/TtsEngine.h
+++ b/include/tts/TtsEngine.h
@@ -53,7 +53,8 @@
     TTS_FEATURE_UNSUPPORTED     = -2,
     TTS_VALUE_INVALID           = -3,
     TTS_PROPERTY_UNSUPPORTED    = -4,
-    TTS_PROPERTY_SIZE_TOO_SMALL = -5
+    TTS_PROPERTY_SIZE_TOO_SMALL = -5,
+    TTS_MISSING_RESOURCES       = -6
 };
 
 class TtsEngine