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
@@ -435,8 +435,14 @@
                              self.log_suffix)
             return None
 
-        d = self.schema.to_dict(tree, validation='skip')
-        d = d or {}  # it may be None if the document is empty but for the root
+        res = self.schema.to_dict(tree, validation='lax')
+        if res is None:
+            d = {}
+        else:
+            (d, errors) = res  # TODO: do something with these errors
+            # d may be None if the document is empty but for the root
+            d = d or {}
+
         metadata = self._translate_dict(d, normalize=False)
         metadata[SCHEMA_URI+'codeRepository'] = self.parse_repositories(d)
         metadata[SCHEMA_URI+'license'] = self.parse_licenses(d)
@@ -466,26 +472,21 @@
         repositories = d.get(POM_PREFIX + 'repositories')
         if not repositories:
             results = [self.parse_repository(d, self._default_repository)]
-        elif isinstance(repositories, dict):
+        else:
             repositories = repositories.get(POM_PREFIX + 'repository') or []
-            if not isinstance(repositories, list):
-                repositories = [repositories]
             results = [self.parse_repository(d, repo)
                        for repo in repositories]
-        else:
-            results = []
         return [res for res in results if res] or None
 
     def parse_repository(self, d, repo):
-        if not isinstance(repo, dict):
+        if not repo:
             return
         if repo.get('layout', 'default') != 'default':
             return  # TODO ?
         url = repo.get(POM_PREFIX + 'url')
         group_id = d.get(POM_PREFIX + 'groupId')
         artifact_id = d.get(POM_PREFIX + 'artifactId')
-        if (isinstance(url, str) and isinstance(group_id, str)
-                and isinstance(artifact_id, str)):
+        if url and group_id and artifact_id:
             repo = os.path.join(url, *group_id.split('.'), artifact_id)
             return {"@id": repo}
 
@@ -495,7 +496,7 @@
         >>> MavenMapping().normalize_groupId('org.example')
         {'@id': 'org.example'}
         """
-        if isinstance(id_, str):
+        if id_:
             return {"@id": id_}
 
     def parse_licenses(self, d):
@@ -550,18 +551,11 @@
          {'@id': 'https://opensource.org/licenses/MIT'}]
         """  # noqa: E501
 
-        licenses = d.get(POM_PREFIX + 'licenses')
-        if not isinstance(licenses, dict):
-            return
-        licenses = licenses.get(POM_PREFIX + 'license')
-        if isinstance(licenses, dict):
-            licenses = [licenses]
-        elif not isinstance(licenses, list):
-            return
+        licenses = d.get(POM_PREFIX + 'licenses') or {}
+        licenses = licenses.get(POM_PREFIX + 'license') or []
         return [{"@id": license[POM_PREFIX + 'url']}
                 for license in licenses
-                if isinstance(license, dict)
-                and isinstance(license.get(POM_PREFIX + 'url'), str)] or None
+                if license[POM_PREFIX + 'url']] or None
 
 
 _normalize_pkginfo_key = str.lower