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
@@ -272,7 +272,8 @@
             return None
 
     def normalize_homepage(self, s):
-        return {"@id": s}
+        if isinstance(s, str):
+            return {"@id": s}
 
 
 @register_mapping
@@ -515,7 +516,8 @@
             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):
@@ -528,10 +530,13 @@
                     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
@@ -769,6 +769,15 @@
     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)
@@ -776,6 +785,16 @@
             '@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 = RevisionMetadataTestIndexer()