diff --git a/codearchiver/core.py b/codearchiver/core.py
index 945b991..bb64fed 100644
--- a/codearchiver/core.py
+++ b/codearchiver/core.py
@@ -84,31 +84,45 @@ class IndexField:
 class Index(list[tuple[str, str]]):
 	'''An index (key-value mapping, possibly with repeated keys) of a file produced by a module'''
 
-	fields: list[IndexField] = []
+	fields: tuple[IndexField] = ()
 	'''The fields for this index'''
 
+	_allFieldsCache: typing.Optional[tuple[IndexField]] = None
+
 	def append(self, *args):
 		if len(args) == 1:
 			args = args[0]
 		return super().append(args)
 
+	# This should be a @classmethod, too, but that's deprecated since Python 3.11.
+	@property
+	def _allFields(self):
+		'''All fields known by this index, own ones and all from superclasses'''
+
+		if type(self)._allFieldsCache is None:
+			fields = []
+			for cls in reversed(type(self).mro()):
+				fields.extend(getattr(cls, 'fields', []))
+			type(self)._allFieldsCache = tuple(fields)
+		return type(self)._allFieldsCache
+
 	def validate(self):
 		'''Check that all keys and values in the index conform to the specification'''
 
 		keyCounts = collections.Counter(key for key, _ in self)
 		keys = set(keyCounts)
 
-		permittedKeys = set(field.key for field in type(self).fields)
+		permittedKeys = set(field.key for field in self._allFields)
 		unrecognisedKeys = keys - permittedKeys
 		if unrecognisedKeys:
 			raise IndexValidationError(f'Unrecognised key(s): {", ".join(sorted(unrecognisedKeys))}')
 
-		requiredKeys = set(field.key for field in type(self).fields if field.required)
+		requiredKeys = set(field.key for field in self._allFields if field.required)
 		missingRequiredKeys = requiredKeys - keys
 		if missingRequiredKeys:
 			raise IndexValidationError(f'Missing required key(s): {", ".join(sorted(missingRequiredKeys))}')
 
-		repeatableKeys = set(field.key for field in type(self).fields if field.repeatable)
+		repeatableKeys = set(field.key for field in self._allFields if field.repeatable)
 		repeatedKeys = set(key for key, count in keyCounts.items() if count > 1)
 		repeatedUnrepeatableKeys = repeatedKeys - repeatableKeys
 		if repeatedUnrepeatableKeys:
diff --git a/codearchiver/modules/git.py b/codearchiver/modules/git.py
index fda28e5..a3e5e98 100644
--- a/codearchiver/modules/git.py
+++ b/codearchiver/modules/git.py
@@ -13,12 +13,12 @@ _logger = logging.getLogger(__name__)
 
 
 class GitIndex(codearchiver.core.Index):
-	fields = [
+	fields = (
 		codearchiver.core.IndexField(key = 'Based on bundle', required = False, repeatable = True),
 		codearchiver.core.IndexField(key = 'Ref', required = True, repeatable = True),
 		codearchiver.core.IndexField(key = 'Root commit', required = True, repeatable = True),
 		codearchiver.core.IndexField(key = 'Commit', required = False, repeatable = True),
-	]
+	)
 
 
 class Git(codearchiver.core.Module):