diff --git a/qutebrowser/completion/completionwidget.py b/qutebrowser/completion/completionwidget.py
index 6fcec9c04..adc5f16ad 100644
--- a/qutebrowser/completion/completionwidget.py
+++ b/qutebrowser/completion/completionwidget.py
@@ -234,6 +234,8 @@ class CompletionView(QTreeView):
             idx = self._next_category_idx(upwards=False)
         elif which == 'prev-category':
             idx = self._next_category_idx(upwards=True)
+        else:  # pragma: no cover
+            raise ValueError("Invalid 'which' value {!r}".format(which))
 
         if not idx.isValid():
             return