Fix/frequency response #569

What does this MR do and why?

Fixes #569 (closed)

• refs #569 (closed) fixing frequency length 2 error
• refs #569 (closed) adding test

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• Includes tests for added/modified code
• Added or modified the documentation if relevant
• Docstrings added/modified for new/modified functions