diff --git a/js/ui/unlockDialog.js b/js/ui/unlockDialog.js
index aea7b6004..4f461f398 100644
--- a/js/ui/unlockDialog.js
+++ b/js/ui/unlockDialog.js
@@ -647,10 +647,14 @@ var UnlockDialog = GObject.registerClass({
         const themeContext = St.ThemeContext.get_for_stage(global.stage);
 
         for (const widget of this._backgroundGroup) {
-            widget.get_effect('blur').set({
-                brightness: BLUR_BRIGHTNESS,
-                sigma: BLUR_SIGMA * themeContext.scale_factor,
-            });
+            const effect = widget.get_effect('blur');
+
+            if (effect) {
+                effect.set({
+                    brightness: BLUR_BRIGHTNESS,
+                    sigma: BLUR_SIGMA * themeContext.scale_factor,
+                });
+            }
         }
     }