Hey Jim,
I tested your code on my test site and it works fine (see attachment). Would you mind testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains. If it’s fixed, you’ll know a plugin caused the problem, and you can narrow down which one by reactivating them one at a time.
If that doesn’t help, please give us access to your WordPress admin so we could take a closer look.
Thanks.