diff --git a/verify_sync.py b/verify_sync.py
index 5518a2c..9e516b1 100644
--- a/verify_sync.py
+++ b/verify_sync.py
@@ -218,7 +218,8 @@ class VerifyWindow(QtWidgets.QMainWindow):
 
         self._setup_ui()
         self._draw_waveforms()
-        self._setup_mpv()
+        # Defer mpv setup until window is shown and winId() is valid
+        QtCore.QTimer.singleShot(0, self._setup_mpv)
 
     def _setup_ui(self):
         central = QtWidgets.QWidget()