diff --git a/tilecloud_chain/__init__.py b/tilecloud_chain/__init__.py
index 728bda535..05c3cc7a8 100644
--- a/tilecloud_chain/__init__.py
+++ b/tilecloud_chain/__init__.py
@@ -208,7 +208,7 @@ class Run:
     def __init__(
         self,
         gene: "TileGeneration",
-        functions: list[Callable[[Tile], Tile]],
+        functions: list[Callable[[Tile], Tile | None]],
         out: IO[str] | None = None,
     ) -> None:
         self.gene = gene
@@ -250,6 +250,7 @@ def __call__(self, tile: Tile | None) -> Tile | None:
                         tile = func(tile)
                     except Exception as e:  # pylint: disable=broad-exception-caught
                         _LOGGER.exception("[%s] Fail to process function %s", tilecoord, func)
+                        assert tile is not None
                         tile.error = e
                 else:
                     tile = func(tile)
@@ -447,9 +448,9 @@ def __init__(
         self._close_actions: list[Close] = []
         self.error_lock = threading.Lock()
         self.error_files_: dict[str, TextIO] = {}
-        self.functions_tiles: list[Callable[[Tile], Tile]] = []
-        self.functions_metatiles: list[Callable[[Tile], Tile]] = []
-        self.functions = self.functions_metatiles
+        self.functions_tiles: list[Callable[[Tile], Tile | None]] = []
+        self.functions_metatiles: list[Callable[[Tile], Tile | None]] = []
+        self.functions: list[Callable[[Tile], Tile | None]] = self.functions_metatiles
         self.metatilesplitter_thread_pool: ThreadPoolExecutor | None = None
         self.multi_thread = multi_thread
         self.maxconsecutive_errors = maxconsecutive_errors
@@ -1367,16 +1368,19 @@ def imap(self, func: Any, time_message: str | None = None) -> None:
         class Func:
             """Function with an additional field used to names it in timing messages."""
 
-            def __init__(self, func: Callable[[Tile], Tile], time_message: str | None) -> None:
+            def __init__(self, func: Callable[[Tile], Tile | None], time_message: str | None) -> None:
                 self.func = func
                 self.time_message = time_message
 
-            def __call__(self, tile: Tile) -> Tile:
+            def __call__(self, tile: Tile) -> Tile | None:
                 return self.func(tile)
 
             def __str__(self) -> str:
                 return f"Func: {self.func}"
 
+            def __repr__(self) -> str:
+                return f"Func: {self.func!r}"
+
         self.functions.append(Func(func, time_message))
 
     def consume(self, test: int | None = None) -> None:
diff --git a/tilecloud_chain/multitilestore.py b/tilecloud_chain/multitilestore.py
index cef73bc61..e237d6ffd 100644
--- a/tilecloud_chain/multitilestore.py
+++ b/tilecloud_chain/multitilestore.py
@@ -25,6 +25,12 @@ def _get_store(self, config_file: str, layer: str) -> TileStore | None:
             self.stores[(config_file, layer)] = store
         return store
 
+    def _get_store_tile(self, tile: Tile) -> TileStore | None:
+        """Return the store corresponding to the tile."""
+        layer = tile.metadata["layer"]
+        config_file = tile.metadata["config_file"]
+        return self._get_store(config_file, layer)
+
     def __contains__(self, tile: Tile) -> bool:
         """
         Return true if this store contains ``tile``.
@@ -32,9 +38,7 @@ def __contains__(self, tile: Tile) -> bool:
         Arguments:
             tile: Tile
         """
-        layer = tile.metadata["layer"]
-        config_file = tile.metadata["config_file"]
-        store = self._get_store(config_file, layer)
+        store = self._get_store_tile(tile)
         assert store is not None
         return tile in store
 
@@ -45,9 +49,7 @@ def delete_one(self, tile: Tile) -> Tile:
         Arguments:
             tile: Tile
         """
-        layer = tile.metadata["layer"]
-        config_file = tile.metadata["config_file"]
-        store = self._get_store(config_file, layer)
+        store = self._get_store_tile(tile)
         assert store is not None
         return store.delete_one(tile)
 
@@ -64,9 +66,7 @@ def put_one(self, tile: Tile) -> Tile:
         Arguments:
             tile: Tile
         """
-        layer = tile.metadata["layer"]
-        config_file = tile.metadata["config_file"]
-        store = self._get_store(config_file, layer)
+        store = self._get_store_tile(tile)
         assert store is not None
         return store.put_one(tile)
 
@@ -77,9 +77,7 @@ def get_one(self, tile: Tile) -> Tile | None:
         Arguments:
             tile: Tile
         """
-        layer = tile.metadata["layer"]
-        config_file = tile.metadata["config_file"]
-        store = self._get_store(config_file, layer)
+        store = self._get_store_tile(tile)
         assert store is not None
         return store.get_one(tile)
 
@@ -117,13 +115,13 @@ def apply(key: tuple[str, str], tiles: Iterator[Tile]) -> Iterator[Tile]:
     def __str__(self) -> str:
         """Return a string representation of the object."""
         stores = {str(store) for store in self.stores.values()}
-        keys = {f"{config_file}/{layer}" for config_file, layer in self.stores}
+        keys = {f"{config_file}:{layer}" for config_file, layer in self.stores}
         return f"{self.__class__.__name__}({', '.join(stores)} - {', '.join(keys)})"
 
     def __repr__(self) -> str:
         """Return a string representation of the object."""
         stores = {repr(store) for store in self.stores.values()}
-        keys = {f"{config_file}/{layer}" for config_file, layer in self.stores}
+        keys = {f"{config_file}:{layer}" for config_file, layer in self.stores}
         return f"{self.__class__.__name__}({', '.join(stores)} - {', '.join(keys)})"
 
     @staticmethod