diff --git a/src/pymor/discretizers/builtin/gui/jupyter/matplotlib.py b/src/pymor/discretizers/builtin/gui/jupyter/matplotlib.py
index f4be348c5af44c7fd3c15a1c5cbb6b400b011dd4..83ba3b7c772aca283ca83403311bca72e3383a1c 100644
--- a/src/pymor/discretizers/builtin/gui/jupyter/matplotlib.py
+++ b/src/pymor/discretizers/builtin/gui/jupyter/matplotlib.py
@@ -38,7 +38,7 @@ class MPLPlotBase:
         if not config.HAVE_IPYWIDGETS and len(U[0]) > 1:
             raise ImportError('cannot visualize: import of ipywidgets failed')
         self.legend = (legend,) if isinstance(legend, str) else legend
-        assert legend is None or isinstance(legend, tuple) and len(legend) == len(U)
+        assert self.legend is None or isinstance(self.legend, tuple) and len(self.legend) == len(U)
         self._set_limits(U)
 
         self.plots = []