diff --git a/swh/indexer/metadata_dictionary.py b/swh/indexer/metadata_dictionary.py
--- a/swh/indexer/metadata_dictionary.py
+++ b/swh/indexer/metadata_dictionary.py
@@ -41,7 +41,11 @@
     >>> merge_values({'@list': ['a', 'b']}, {'@list': ['c']})
     {'@list': ['a', 'b', 'c']}
     """
-    if isinstance(v1, dict) and set(v1) == {'@list'}:
+    if v1 is None:
+        return v2
+    elif v2 is None:
+        return v1
+    elif isinstance(v1, dict) and set(v1) == {'@list'}:
         assert isinstance(v1['@list'], list)
         if isinstance(v2, dict) and set(v2) == {'@list'}:
             assert isinstance(v2['@list'], list)
@@ -325,7 +329,8 @@
         >>> NpmMapping().normalize_homepage('https://example.org/~john.doe')
         {'@id': 'https://example.org/~john.doe'}
         """
-        return {"@id": s}
+        if isinstance(s, str):
+            return {"@id": s}
 
 
 @register_mapping
@@ -591,7 +596,8 @@
             return evaluator(tree.body)
 
     def normalize_homepage(self, s):
-        return {"@id": s}
+        if isinstance(s, str):
+            return {"@id": s}
 
     def normalize_license(self, s):
         if isinstance(s, str):
@@ -604,10 +610,13 @@
                     if isinstance(license, str)]
 
     def normalize_author(self, author):
-        return {"@list": [author]}
+        if isinstance(author, str):
+            return {"@list": [author]}
 
     def normalize_authors(self, authors):
-        return {"@list": authors}
+        if isinstance(authors, list):
+            return {"@list": [author for author in authors
+                              if isinstance(author, str)]}
 
 
 def main():
diff --git a/swh/indexer/tests/test_metadata.py b/swh/indexer/tests/test_metadata.py
--- a/swh/indexer/tests/test_metadata.py
+++ b/swh/indexer/tests/test_metadata.py
@@ -85,12 +85,14 @@
         self.assertEqual(
             merge_values('a', ['b', 'c']),
             ['a', 'b', 'c'])
+
         self.assertEqual(
             merge_values({'@list': ['a']}, {'@list': ['b']}),
             {'@list': ['a', 'b']})
         self.assertEqual(
             merge_values({'@list': ['a', 'b']}, {'@list': ['c']}),
             {'@list': ['a', 'b', 'c']})
+
         with self.assertRaises(ValueError):
             merge_values({'@list': ['a']}, 'b')
         with self.assertRaises(ValueError):
@@ -100,6 +102,22 @@
         with self.assertRaises(ValueError):
             merge_values(['a'], {'@list': ['b']})
 
+        self.assertEqual(
+            merge_values('a', None),
+            'a')
+        self.assertEqual(
+            merge_values(['a', 'b'], None),
+            ['a', 'b'])
+        self.assertEqual(
+            merge_values(None, ['b', 'c']),
+            ['b', 'c'])
+        self.assertEqual(
+            merge_values({'@list': ['a']}, None),
+            {'@list': ['a']})
+        self.assertEqual(
+            merge_values(None, {'@list': ['a']}),
+            {'@list': ['a']})
+
     def test_compute_metadata_none(self):
         """
         testing content empty content is empty
@@ -808,6 +826,15 @@
     def test_gemspec_invalid_author(self):
         raw_content = b"""
 Gem::Specification.new do |s|
+  s.author      = ["Ruby Coder"]
+end"""
+        result = MAPPINGS['GemspecMapping'].translate(raw_content)
+        self.assertEqual(result, {
+            '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
+            'type': 'SoftwareSourceCode',
+        })
+        raw_content = b"""
+Gem::Specification.new do |s|
   s.author      = "Ruby Coder1",
 end"""
         result = MAPPINGS['GemspecMapping'].translate(raw_content)
@@ -815,6 +842,16 @@
             '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
             'type': 'SoftwareSourceCode',
         })
+        raw_content = b"""
+Gem::Specification.new do |s|
+  s.authors     = ["Ruby Coder1", ["Ruby Coder2"]]
+end"""
+        result = MAPPINGS['GemspecMapping'].translate(raw_content)
+        self.assertEqual(result, {
+            '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
+            'type': 'SoftwareSourceCode',
+            'author': ['Ruby Coder1'],
+        })
 
     def test_revision_metadata_indexer(self):
         metadata_indexer = RevisionMetadataIndexer(