Need a better error message

Ett AJAX HTTP-fel inträffade
Resultatkod för HTTP: 500
Felsökningsinformation följer.
Sökväg: /drupal/?q=system/ajax
Statustext: Service unavailable (with message)
Svarstext: Recoverable fatal error: Argument 2 passed to drupal_array_get_nested_value() must be an array, null given, called in /home/humanist/public_html/drupal/modules/field/field.form.inc on line 363 and defined i drupal_array_get_nested_value() (rad 6508 av /home/humanist/public_html/drupal/includes/common.inc).

I got the message when I clicked on Pick after I had chosen a relation type. The reason was that I had not chosed a person.