diff --git a/telebot/asyncio_filters.py b/telebot/asyncio_filters.py
index 312b290..c594d31 100644
--- a/telebot/asyncio_filters.py
+++ b/telebot/asyncio_filters.py
@@ -10,8 +10,12 @@ class SimpleCustomFilter(ABC):
     Simple Custom Filter base class.
     Create child class with check() method.
     Accepts only message, returns bool value, that is compared with given in handler.
+    
+    Child classes should have .key property.
     """
 
+    key: str = None
+
     async def check(self, message):
         """
         Perform a check.
@@ -26,8 +30,12 @@ class AdvancedCustomFilter(ABC):
     Accepts two parameters, returns bool: True - filter passed, False - filter failed.
     message: Message class
     text: Filter value given in handler
+
+    Child classes should have .key property.
     """
 
+    key: str = None
+
     async def check(self, message, text):
         """
         Perform a check.
diff --git a/telebot/custom_filters.py b/telebot/custom_filters.py
index 1bd80b3..abeffdd 100644
--- a/telebot/custom_filters.py
+++ b/telebot/custom_filters.py
@@ -12,8 +12,12 @@ class SimpleCustomFilter(ABC):
     Simple Custom Filter base class.
     Create child class with check() method.
     Accepts only message, returns bool value, that is compared with given in handler.
+
+    Child classes should have .key property.
     """
 
+    key: str = None
+
     def check(self, message):
         """
         Perform a check.
@@ -28,8 +32,12 @@ class AdvancedCustomFilter(ABC):
     Accepts two parameters, returns bool: True - filter passed, False - filter failed.
     message: Message class
     text: Filter value given in handler
+
+    Child classes should have .key property.
     """
 
+    key: str = None
+
     def check(self, message, text):
         """
         Perform a check.