diff --git a/src/calibre/gui2/tts/piper.py b/src/calibre/gui2/tts/piper.py
index ed25a33c38..94b760dde6 100644
--- a/src/calibre/gui2/tts/piper.py
+++ b/src/calibre/gui2/tts/piper.py
@@ -355,7 +355,7 @@ class Piper(TTSBackend):
         if err is not None:
             self._errors_from_piper.append(str(err))
             self._errors_from_piper.append(tb)
-        else:
+        elif self._utterances_being_synthesized:
             u = self._utterances_being_synthesized[0]
             if u.id == sr.utterance_id:
                 u.audio_data.append(sr.audio_data)