diff --git a/b/cntoai/model_usage_admin_report.dspy b/b/cntoai/model_usage_admin_report.dspy
index 5a6355e..6f7af45 100644
--- a/b/cntoai/model_usage_admin_report.dspy
+++ b/b/cntoai/model_usage_admin_report.dspy
@@ -127,6 +127,10 @@ def _format_amount_summary(summary):
     return formatted
 
 
+def _usage_time_sort_key(item):
+    return str(item.get('last_usage_time') or '')
+
+
 def _normalize_usage_row(row, bill_amount_map=None):
     usage = _parse_usage_content(row.get('usage_content'))
     orderid = row.get('orderid')
@@ -341,7 +345,7 @@ def _aggregate_admin_summary(items, user_map, org_map):
                 or str(usage_time) < str(bucket['first_usage_time'])
             ):
                 bucket['first_usage_time'] = usage_time
-    return sorted(buckets.values(), key=lambda x: x['amount'], reverse=True)
+    return sorted(buckets.values(), key=_usage_time_sort_key, reverse=True)
 
 
 def _aggregate_items(items, group_by=None):