diff --git a/src/registry.py b/src/registry.py
index 9e3e0c1..84a714b 100644
--- a/src/registry.py
+++ b/src/registry.py
@@ -195,6 +195,8 @@ def get_index(self, allowed_media_type: Optional[list[str]] = None):
         try:
             self._check_200_response(response)
             index = response.json()
+            # Ensure mediaType is set for existing indices
+            index["mediaType"] = "application/vnd.oci.image.index.v1+json"
             return index
 
         except ValueError:
diff --git a/src/schemas.py b/src/schemas.py
index dcb5624..b6306af 100644
--- a/src/schemas.py
+++ b/src/schemas.py
@@ -33,6 +33,7 @@
     "required": [
         "schemaVersion",
         "manifests",
+        "mediaType",
     ],
     "properties": indexProperties,
     "additionalProperties": True,