diff --git a/lexos/models/rolling_window_model.py b/lexos/models/rolling_window_model.py
index 02799517a..20a087a64 100644
--- a/lexos/models/rolling_window_model.py
+++ b/lexos/models/rolling_window_model.py
@@ -711,7 +711,7 @@ def _add_milestone(self, result_plot: List[go.Scattergl]) -> go.Figure:
             self._find_mile_stone_windows_indexes_in_all_windows()
 
         # Check if passed in mile stones exist in the file.
-        if milestones_dict is not {}:
+        if milestones_dict != {}:
             # Find max and min y value in the result plot.
             y_max_in_each_plot = \
                 [max(each_plot['y'][~np.isnan(each_plot['y'])])