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
@@ -472,12 +472,17 @@
          {'@id': 'https://opensource.org/licenses/MIT'}]
         """
 
-        licenses = d.get('licenses', {}).get('license', [])
+        licenses = d.get('licenses')
+        if not isinstance(licenses, dict):
+            return
+        licenses = licenses.get('license')
         if isinstance(licenses, dict):
             licenses = [licenses]
+        elif not isinstance(licenses, list):
+            return
         return [{"@id": license['url']}
                 for license in licenses
-                if 'url' in license] or None
+                if isinstance(license, dict) and 'url' in license] or None
 
 
 _normalize_pkginfo_key = str.lower
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
@@ -707,6 +707,50 @@
             'https://repo.maven.apache.org/maven2/com/mycompany/app/my-app',
         })
 
+        raw_content = b"""
+        <project>
+          <name>Maven Default Project</name>
+          <modelVersion>4.0.0</modelVersion>
+          <groupId>com.mycompany.app</groupId>
+          <artifactId>my-app</artifactId>
+          <version>1.2.3</version>
+          <licenses>
+          </licenses>
+        </project>"""
+        result = self.maven_mapping.translate(raw_content)
+        self.assertEqual(result, {
+            '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
+            'type': 'SoftwareSourceCode',
+            'name': 'Maven Default Project',
+            'identifier': 'com.mycompany.app',
+            'version': '1.2.3',
+            'codeRepository':
+            'https://repo.maven.apache.org/maven2/com/mycompany/app/my-app',
+        })
+
+    def test_compute_metadata_maven_invalid_licenses(self):
+        raw_content = b"""
+        <project>
+          <name>Maven Default Project</name>
+          <modelVersion>4.0.0</modelVersion>
+          <groupId>com.mycompany.app</groupId>
+          <artifactId>my-app</artifactId>
+          <version>1.2.3</version>
+          <licenses>
+            foo
+          </licenses>
+        </project>"""
+        result = self.maven_mapping.translate(raw_content)
+        self.assertEqual(result, {
+            '@context': 'https://doi.org/10.5063/schema/codemeta-2.0',
+            'type': 'SoftwareSourceCode',
+            'name': 'Maven Default Project',
+            'identifier': 'com.mycompany.app',
+            'version': '1.2.3',
+            'codeRepository':
+            'https://repo.maven.apache.org/maven2/com/mycompany/app/my-app',
+        })
+
     def test_compute_metadata_maven_multiple(self):
         '''Tests when there are multiple code repos and licenses.'''
         raw_content = b"""