diff --git a/platypush/plugins/assistant/__init__.py b/platypush/plugins/assistant/__init__.py
index 6291b111..658192db 100644
--- a/platypush/plugins/assistant/__init__.py
+++ b/platypush/plugins/assistant/__init__.py
@@ -85,6 +85,7 @@ class AssistantPlugin(Plugin, AssistantEntityManager, ABC):
         super().__init__(*args, **kwargs)
         self.tts_plugin = tts_plugin
         self.tts_plugin_args = tts_plugin_args or {}
+        self._conversation_start_sound = None
         if conversation_start_sound:
             self._conversation_start_sound = os.path.abspath(
                 os.path.expanduser(conversation_start_sound)