diff --git a/src/registry.py b/src/registry.py
index 9e3e0c1..2371ed1 100644
--- a/src/registry.py
+++ b/src/registry.py
@@ -461,6 +461,10 @@ def push_image_manifest(
 
     def update_index(self, manifest_folder):
         index = self.get_index()
+        # Ensure mediaType is set for existing indices
+        if "mediaType" not in index:
+            index["mediaType"] = "application/vnd.oci.image.index.v1+json"
+
         new_entries = 0
 
         for file in os.listdir(manifest_folder):
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,