diff --git a/src/caoscrawler/converters.py b/src/caoscrawler/converters.py
index 5a3f4a090785f762cad49054570d15a62649bfbe..5dba0726bba5351cb75ab363985daa639a6e56ac 100644
--- a/src/caoscrawler/converters.py
+++ b/src/caoscrawler/converters.py
@@ -365,10 +365,10 @@ class Converter(object, metaclass=ABCMeta):
 
         Parameters:
         ------------
-        
+
         values: GeneralStore
             The GeneralStore to store values in.
-        
+
         element: StructureElement
             The StructureElement to extract values from.
         """
@@ -382,11 +382,11 @@ class Converter(object, metaclass=ABCMeta):
                            transformer_functions: dict):
         """
         Check if transformers are defined using the "transform" keyword.
-        The apply the transformers to the variables defined in GeneralStore "values".
+        Then apply the transformers to the variables defined in GeneralStore "values".
 
         Parameters:
         ------------
-        
+
         values: GeneralStore
             The GeneralStore to store values in.
 
@@ -395,6 +395,9 @@ class Converter(object, metaclass=ABCMeta):
         """
 
         if "transform" in self.definition:
+            if not isinstance(self.definition["transform"], dict):
+                raise RuntimeError("The value corresponding to the 'transform' key in the "
+                                   "converter definition must be a dict")
             for transformer_key, transformer in self.definition["transform"].items():
                 if "in" not in transformer:
                     raise RuntimeError("In-variable not defined!")
@@ -402,7 +405,10 @@ class Converter(object, metaclass=ABCMeta):
                     raise RuntimeError("Out-variable not defined!")
                 if "functions" not in transformer:
                     raise RuntimeError("No functions given for transformer!")
-                
+                if not isinstance(self.definition["functions"], list):
+                    raise RuntimeError("The value corresponding to the 'functions' key in the "
+                                       "transform section must be a dict")
+
                 # Currently overwriting the input variable would be possible.
                 # Shall we prevent that?
                 # Forbid overwriting existing variables at all?
@@ -414,7 +420,8 @@ class Converter(object, metaclass=ABCMeta):
 
                 for tr_func_el in transformer["functions"]:
                     if not isinstance(tr_func_el, dict):
-                        raise RuntimeError("List elements must be dictonaries!")
+                        raise RuntimeError("Elements of the list of the functions key "
+                                           "must be dictonaries!")
                     if len(tr_func_el) != 1:
                         raise RuntimeError("List element dictionaries must have exactly"
                                            " one element with they key being the name"
diff --git a/src/caoscrawler/scanner.py b/src/caoscrawler/scanner.py
index 6169de213e4a2d33e0329d4c5ed9299392410d2d..fd82a896d88d6470cfe0883fcf85523892e01ae3 100644
--- a/src/caoscrawler/scanner.py
+++ b/src/caoscrawler/scanner.py
@@ -36,21 +36,19 @@ import importlib
 import logging
 import os
 import warnings
-import yaml
+from typing import Any, Optional, Type, Union
 
+import caosdb as db
+import yaml
 from importlib_resources import files
 from jsonschema import validate
-from typing import Any, Optional, Type, Union
 
-import caosdb as db
 from .converters import Converter
-
+from .debug_tree import DebugTree
 from .stores import GeneralStore, RecordStore
-from .structure_elements import StructureElement, Directory
+from .structure_elements import Directory, StructureElement
 from .version import check_cfood_version
 
-from .debug_tree import DebugTree
-
 logger = logging.getLogger(__name__)
 
 
@@ -255,8 +253,6 @@ def scanner(items: list[StructureElement],
             restricted_path: Optional[list[str]] = None,
             crawled_data: Optional[list[db.Record]] = None,
             debug_tree: Optional[DebugTree] = None,
-            # This would fit better after converters parameter (non-optional),
-            # but is added here for backwards compatibility.
             registered_transformer_functions: Optional[dict] = None):
     """Crawl a list of StructureElements and apply any matching converters.
 
@@ -284,14 +280,14 @@ def scanner(items: list[StructureElement],
         A dictionary of transformer functions that can be used in the "transform" block
         of a converter and that allows to apply simple transformations to variables extracted
         either by the current converter or to other variables found in the current variable store.
-    
+
         Each function is a dictionary:
         - The key is the name of the function to be looked up in the dictionary
           of registered transformer functions.
         - The value is a dictionary with key, value-assignments which will be
           passed to the transformer function.
         The transformer function needs to be of the form:
-    
+
         def func(in_value: Any, in_parameters: dict) -> Any:
             pass