diff --git a/src/pyside6/trade_widget.py b/src/pyside6/trade_widget.py
index 18b435f..71a0a24 100644
--- a/src/pyside6/trade_widget.py
+++ b/src/pyside6/trade_widget.py
@@ -62,6 +62,7 @@ class TradeCollection(QWidget):
                  parent):
         super(TradeCollection, self).__init__()
         self.parent = parent
+        assert message.trade
         self.unique_item = message.trade.unique_item()
         self.main_layout = QVBoxLayout()
         self.setLayout(self.main_layout)