diff --git a/src/pipecat/services/speechmatics/stt.py b/src/pipecat/services/speechmatics/stt.py
index d6c947d38..6859306d3 100644
--- a/src/pipecat/services/speechmatics/stt.py
+++ b/src/pipecat/services/speechmatics/stt.py
@@ -191,7 +191,7 @@ class SpeakerFragments:
             passive_format = active_format
         return {
             "text": self._format_text(active_format if self.is_active else passive_format),
-            "user_id": self.speaker_id,
+            "user_id": self.speaker_id or "",
             "timestamp": self.timestamp,
             "language": self.language,
             "result": [frag.result for frag in self.fragments],