diff --git a/bin/i3-companion b/bin/i3-companion
index 908d84a..cf2474d 100755
--- a/bin/i3-companion
+++ b/bin/i3-companion
@@ -420,7 +420,7 @@ async def workspace_exclusive(i3, event):
     w = event.container
 
     # Can the current window intrude the workspace?
-    if w.floating in {"auto_on", "user_on"} or w.window_class.lower in intrusive_apps:
+    if w.floating in {"auto_on", "user_on"} or w.window_class.lower() in intrusive_apps:
         logger.debug("window %s can intrude", w.window_class)
         return