diff --git a/src/stores/keys.rs b/src/stores/keys.rs
index 07cc399..32bfcfe 100644
--- a/src/stores/keys.rs
+++ b/src/stores/keys.rs
@@ -210,7 +210,10 @@ impl KeysPersistentStorage for MongoPersistentStorage {
     async fn get_cacao_by_identity_key(&self, identity_key: &str) -> Result<Cacao, StoreError> {
         info!("get_cacao_by_identity_key");
         let filter = doc! {
-            "identities.identity_key": identity_key,
+            "identities.identity_key": {
+                "$exists": true, // https://docs.aws.amazon.com/documentdb/latest/developerguide/functional-differences.html#functional-differences.sparse-index
+                "$eq": identity_key,
+            },
         };
 
         info!("constructing not_found");