diff --git a/qt/python/mantidqt/widgets/plotconfigdialog/legendtabwidget/presenter.py b/qt/python/mantidqt/widgets/plotconfigdialog/legendtabwidget/presenter.py
index 1628fca82994c54a4c31d6d93fd3443d9e30ebca..d93aac576de27500dffe04ff5d8d227dafe7c4ee 100644
--- a/qt/python/mantidqt/widgets/plotconfigdialog/legendtabwidget/presenter.py
+++ b/qt/python/mantidqt/widgets/plotconfigdialog/legendtabwidget/presenter.py
@@ -56,7 +56,7 @@ class LegendTabWidgetPresenter:
 
         # Converts alpha value (opacity value between 0 and 1) to transparency percentage.
         if int(matplotlib.__version__[0]) >= 2:
-            transparency = 100-(legend_props.transparency*100)
+            transparency = int(100 - (legend_props.transparency*100))
             self.view.set_transparency_spin_box(transparency)
             self.view.set_transparency_slider(transparency)
         self.view.set_entries_font(legend_props.entries_font)