diff --git a/src/caoscrawler/scanner.py b/src/caoscrawler/scanner.py
index 18d722ed8c485dc0472344407f345a7a0e517b60..3f8b85f4c49b33d4e065a840293c871381c31396 100644
--- a/src/caoscrawler/scanner.py
+++ b/src/caoscrawler/scanner.py
@@ -36,6 +36,7 @@ import importlib
 import logging
 import os
 import warnings
+from collections.abc import Callable
 from typing import Any, Optional, Type, Union
 
 import caosdb as db
@@ -181,7 +182,7 @@ def create_converter_registry(definition: dict):
     return converter_registry
 
 
-def create_transformer_registry(definition: dict):
+def create_transformer_registry(definition: dict[str, dict[str, str]]):
     """
     Currently the transformer registry is a dictionary containing for each transformer:
     - key is the short code, abbreviation for the converter class name
@@ -201,7 +202,7 @@ def create_transformer_registry(definition: dict):
     with open(str(files('caoscrawler').joinpath('default_transformers.yml')), "r") as f:
         transformer_def: dict[str, dict[str, str]] = yaml.safe_load(f)
 
-    registry = {}
+    registry: dict[str, Callable[[Any, dict], Any]] = {}
     # More transformers from definition file:
     if "Transformers" in definition:
         for key, entry in definition["Transformers"].items():