Fix check in member

Description

Fixed an incorrect use of at[] by check_in_member. at[] does not support a combination of integer and index lookups.

Related Issue(s)

Changes Made

• Change 1: changed at[] to the correct method.

Screenshots / Snippets (if applicable)

Test Plan

• make test

Checklist

• Code follows the project's coding style and conventions.
• Automated tests for the changes have been added or existing tests have been updated.
• Code coverage for new or modified code is maintained or improved.
• The code builds and runs without errors or warnings.
• Documentation has been updated to reflect the changes made.
• All discussions and feedback on the PR have been addressed.
• CI/CD pipelines pass successfully, including any required checks or tests.
• The PR has been reviewed and approved by the designated reviewers.