diff --git a/manimlib/mobject/mobject.py b/manimlib/mobject/mobject.py
index 16a4fc5c..8ef3625b 100644
--- a/manimlib/mobject/mobject.py
+++ b/manimlib/mobject/mobject.py
@@ -1597,7 +1597,7 @@ class Group(Mobject):
         self.add(*mobjects)
         
     def __add__(self, other : 'Mobject' or 'Group'):
-        assert(isinstance(other(Mobject))
+        assert(isinstance(other, Mobject))
         return self.add(other)