make have-len more strict, only accept int

[aelsabbahy]

Checklist

• [ ] make test-all (UNIX) passes. CI will also test this
• [ ] unit and/or integration tests are included (if applicable)
• [ ] documentation is changed or added (if applicable)

Description of change

Follow up to #849, merged the PR too soon.

[aelsabbahy]

nvm, json unmarshals numbers into floats. The merged code is the correct one.