diff --git a/kart/gui/mapswipetool.py b/kart/gui/mapswipetool.py
index 84c7478..9ecc957 100644
--- a/kart/gui/mapswipetool.py
+++ b/kart/gui/mapswipetool.py
@@ -35,8 +35,8 @@ def activate(self):
         self.setLayersSwipe()
         self.swipe.setIsVertical(True)
         self.swipe.setLength(
-            self.swipe.boundingRect().width() / 2,
-            self.swipe.boundingRect().height() / 2,
+            int(self.swipe.boundingRect().width() / 2),
+            int(self.swipe.boundingRect().height() / 2),
         )
 
     def deactivate(self):