diff --git a/apis/validator/aggregate_attestation.yaml b/apis/validator/aggregate_attestation.yaml
index 76e6d713..2fe1fcef 100644
--- a/apis/validator/aggregate_attestation.yaml
+++ b/apis/validator/aggregate_attestation.yaml
@@ -7,6 +7,9 @@ get:
     A 503 error must be returned if the block identified by the response
     `beacon_block_root` is optimistic (i.e. the aggregated attestation attests
     to a block that has not been fully verified by an execution engine).
+
+    A 500 error must be returned if no attestation is available for the requested
+    `attestation_data_root`.
   tags:
     - ValidatorRequiredApi
     - Validator