diff --git a/qutebrowser/browser/webengine/webenginetab.py b/qutebrowser/browser/webengine/webenginetab.py
index 204199c72..96c7e562e 100644
--- a/qutebrowser/browser/webengine/webenginetab.py
+++ b/qutebrowser/browser/webengine/webenginetab.py
@@ -211,7 +211,7 @@ class WebEngineScroller(browsertab.AbstractScroller):
         """Update the scroll position attributes when it changed."""
         def update_scroll_pos(jsret):
             """Callback after getting scroll position via JS."""
-            assert isinstance(jsret, dict)
+            assert isinstance(jsret, dict), jsret
             self._pos_perc = (jsret['perc']['x'], jsret['perc']['y'])
             self._pos_px = QPoint(jsret['px']['x'], jsret['px']['y'])
             self.perc_changed.emit(*self._pos_perc)