diff --git a/infra/smalruby-classroom/lambda/handler.ts b/infra/smalruby-classroom/lambda/handler.ts
index 46daa8204d..8030532f9a 100644
--- a/infra/smalruby-classroom/lambda/handler.ts
+++ b/infra/smalruby-classroom/lambda/handler.ts
@@ -3752,14 +3752,17 @@ async function handleListSharedAssignments(
     }
   }
 
+  // DynamoDB rejects an EMPTY ExpressionAttributeNames map, so the key must
+  // be omitted when there is nothing to alias (mine=1 with no filters).
+  const expressionNames = {
+    ...(mine ? {} : { '#status': 'status' }),
+    ...names,
+  };
   const result = await docClient.send(new QueryCommand({
     TableName: SHARED_ASSIGNMENTS_TABLE,
     IndexName: mine ? 'authorSub-createdAt-index' : 'status-createdAt-index',
     KeyConditionExpression: mine ? 'authorSub = :pk' : '#status = :pk',
-    ExpressionAttributeNames: {
-      ...(mine ? {} : { '#status': 'status' }),
-      ...names,
-    },
+    ...(Object.keys(expressionNames).length > 0 ? { ExpressionAttributeNames: expressionNames } : {}),
     ExpressionAttributeValues: {
       ':pk': mine ? identity.sub : 'published',
       ...values,
diff --git a/infra/smalruby-classroom/lambda/tests/handler-shared-assignments.test.ts b/infra/smalruby-classroom/lambda/tests/handler-shared-assignments.test.ts
index 9127b2ead1..e59833fa55 100644
--- a/infra/smalruby-classroom/lambda/tests/handler-shared-assignments.test.ts
+++ b/infra/smalruby-classroom/lambda/tests/handler-shared-assignments.test.ts
@@ -325,6 +325,10 @@ describe('みんなの課題 (issue #1068)', () => {
       }));
       expect(captured?.IndexName).toBe('authorSub-createdAt-index');
       expect((captured?.ExpressionAttributeValues as Record<string, unknown>)[':pk']).toBe('dev-test-teacher');
+      // DynamoDB rejects an EMPTY ExpressionAttributeNames map — with no
+      // filters and no #status alias, the key must be omitted entirely
+      // (prod 500 on 自分の投稿, 2026-07-18).
+      expect(captured?.ExpressionAttributeNames).toBeUndefined();
       expect(JSON.parse(res.body as string).items[0].status).toBe('unlisted');
     });
   });