Prevent user from adding more than one Primary phone field to a profile

Add form rule to block user from adding more than one field in a profile with field type = Phone and is_primary = TRUE. If user attempts to add a second primary phone field - form rule message is:
"Only one primary phone field can be included in a profile."

— Original post —
Create a profile and add a phone field. Then attempt to add a second phone field of a different type (e.g. mobile).

The form never saves, and the JSON returns a 500 error in the console. This is reproducible on the demo site.