diff --git a/lark/indenter.py b/lark/indenter.py
index a4bbb24..1a9e587 100644
--- a/lark/indenter.py
+++ b/lark/indenter.py
@@ -74,31 +74,31 @@ class Indenter(PostLex, ABC):
     @property
     @abstractmethod
     def NL_type(self) -> str:
-        return NotImplemented
+        raise NotImplementedError()
 
     @property
     @abstractmethod
     def OPEN_PAREN_types(self) -> List[str]:
-        return NotImplemented
+        raise NotImplementedError()
 
     @property
     @abstractmethod
     def CLOSE_PAREN_types(self) -> List[str]:
-        return NotImplemented
+        raise NotImplementedError()
 
     @property
     @abstractmethod
     def INDENT_type(self) -> str:
-        return NotImplemented
+        raise NotImplementedError()
 
     @property
     @abstractmethod
     def DEDENT_type(self) -> str:
-        return NotImplemented
+        raise NotImplementedError()
 
     @property
     @abstractmethod
     def tab_len(self) -> int:
-        return NotImplemented
+        raise NotImplementedError()
 
 ###}