Help:Schema/Error handling/NULL value found, but an object is required


 * Error: Error text::NULL value found, but an object is required
 * Type: Error type::Validation error
 * Example: ["constraints", "NULL value found, but an object is required"]

Explanation
The validation schema expects that the  keyword is an object (meaning to be enclosed by   ...  ) as in:

"constraints": { "must_exists": true }