Confusing error message "Cannot read JSON." in HTTP agent item

Steps to reproduce:

1. Add new item
2. Select type as HTTP agent
3. Type name, key, url
4. Select "Request body type" as JSON data (or XML data)
5. Type wrong "Request body"
6. Try create item

Result: confusing error message "Cannot read JSON."
Expected: Something like, Incorrect value for field "Request body":