Date field doesn't save to database for just Year attribute

I have a Drupal-7.14 install and have an entity with a date field attached to it.
I was only collecting the Year attribute with the option for collecting an end date.
I got the following error message when trying to submit the form:

• PDOException: SQLSTATE[22007]: Invalid datetime format: 1292 Incorrect datetime value: '2010' for column 'field_student_enrolled_value' at row 1: INSERT INTO {field_data_field_student_enrolled} (entity_type, entity_id, revision_id, bundle, delta, language, field_student_enrolled_value, field_student_enrolled_value2) VALUES (:db_insert_placeholder_0, :db_insert_placeholder_1, :db_insert_placeholder_2, :db_insert_placeholder_3, :db_insert_placeholder_4, :db_insert_placeholder_5, :db_insert_placeholder_6, :db_insert_placeholder_7); Array ( [:db_insert_placeholder_0] => profile2 [:db_insert_placeholder_1] => 616 [:db_insert_placeholder_2] => 616 [:db_insert_placeholder_3] => student [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => und [:db_insert_placeholder_6] => 2010 [:db_insert_placeholder_7] => ) in field_sql_storage_field_storage_write() (line 448 of /home/clinks/domains/partydev.commonnonsense.co.uk/public_html/modules/field/modules/field_sql_storage/field_sql_storage.module).
  

I tried debugging it and I get the same error with just year and month, but it works fine for year, month and day together.
Curious little error so I thought I'd report it. Anyone get similar?