diff --git a/swh/indexer/metadata_dictionary/__init__.py b/swh/indexer/metadata_dictionary/__init__.py
--- a/swh/indexer/metadata_dictionary/__init__.py
+++ b/swh/indexer/metadata_dictionary/__init__.py
@@ -27,7 +27,7 @@
 @click.command()
 @click.argument("mapping_name")
 @click.argument("file_name")
-def main(mapping_name, file_name):
+def main(mapping_name: str, file_name: str):
     from pprint import pprint
 
     with open(file_name, "rb") as fd:
diff --git a/swh/indexer/metadata_dictionary/base.py b/swh/indexer/metadata_dictionary/base.py
--- a/swh/indexer/metadata_dictionary/base.py
+++ b/swh/indexer/metadata_dictionary/base.py
@@ -5,7 +5,7 @@
 
 import json
 import logging
-from typing import List
+from typing import List, Union
 
 from swh.indexer.codemeta import SCHEMA_URI, compact, merge_values
 
@@ -32,7 +32,7 @@
         raise NotImplementedError(f"{self.__class__.__name__}.name")
 
     @classmethod
-    def detect_metadata_files(cls, files):
+    def detect_metadata_files(cls, files: list):
         """
         Detects files potentially containing metadata
 
@@ -62,7 +62,7 @@
     @classmethod
     def detect_metadata_files(cls, file_entries):
         for entry in file_entries:
-            if entry["name"].lower() == cls.filename.lower():
+            if str(entry["name"]).lower() == (cls.filename).lower():
                 return [entry["sha1"]]
         return []
 
@@ -150,7 +150,7 @@
 class JsonMapping(DictMapping, SingleFileMapping):
     """Base class for all mappings that use a JSON file as input."""
 
-    def translate(self, raw_content):
+    def translate(self, raw_content) -> Union[dict, None]:
         """
         Translates content by parsing content from a bytestring containing
         json data and translating with the appropriate mapping
@@ -165,13 +165,15 @@
         """
         try:
             raw_content = raw_content.decode()
+            return None
         except UnicodeDecodeError:
             self.log.warning("Error unidecoding from %s", self.log_suffix)
-            return
+            return None
         try:
             content_dict = json.loads(raw_content)
+            return None
         except json.JSONDecodeError:
             self.log.warning("Error unjsoning from %s", self.log_suffix)
-            return
+            return None
         if isinstance(content_dict, dict):
             return self._translate_dict(content_dict)