diff --git a/Notes/02_Working_with_data/02_Containers.md b/Notes/02_Working_with_data/02_Containers.md
index 41833d4aa..f757fe514 100644
--- a/Notes/02_Working_with_data/02_Containers.md
+++ b/Notes/02_Working_with_data/02_Containers.md
@@ -159,7 +159,8 @@ Then to access:
 >>>
 ```
 
-*Neither a list, a set, nor another dictionary can serve as a dictionary key, because lists and dictionaries are mutable.*
+*Neither a list, a set, nor another dictionary can serve as a dictionary key, because lists, sets, and dictionaries are mutable.*
+*for example: my_dict = {{ {1, 2, 3}: "value" }}  # Raises TypeError: unhashable type: 'set'*
 
 ### Sets