diff --git a/src/PIL/ImageFile.py b/src/PIL/ImageFile.py
index 4bc70cc76..1bf8a7e5f 100644
--- a/src/PIL/ImageFile.py
+++ b/src/PIL/ImageFile.py
@@ -447,7 +447,7 @@ class StubHandler(abc.ABC):
         pass
 
 
-class StubImageFile(ImageFile):
+class StubImageFile(ImageFile, metaclass=abc.ABCMeta):
     """
     Base class for stub image loaders.
 
@@ -455,9 +455,9 @@ class StubImageFile(ImageFile):
     certain format, but relies on external code to load the file.
     """
 
+    @abc.abstractmethod
     def _open(self) -> None:
-        msg = "StubImageFile subclass must implement _open"
-        raise NotImplementedError(msg)
+        pass
 
     def load(self) -> Image.core.PixelAccess | None:
         loader = self._load()
@@ -471,10 +471,10 @@ class StubImageFile(ImageFile):
         self.__dict__ = image.__dict__
         return image.load()
 
+    @abc.abstractmethod
     def _load(self) -> StubHandler | None:
         """(Hook) Find actual image loader."""
-        msg = "StubImageFile subclass must implement _load"
-        raise NotImplementedError(msg)
+        pass
 
 
 class Parser: