General error: 1366 Incorrect integer value

When I get to the final stages of checkout (after the review page) I get the following error

messagePDOException: SQLSTATE[HY000]: General error: 1366 Incorrect integer value: '' for column 'commerce_unit_price_amount' at row 1: INSERT INTO {field_data_commerce_unit_price} (entity_type, entity_id, revision_id, bundle, delta, language, commerce_unit_price_amount, commerce_unit_price_currency_code, commerce_unit_price_data) 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, :db_insert_placeholder_8); Array ( [:db_insert_placeholder_0] => commerce_line_item [:db_insert_placeholder_1] => 11 [:db_insert_placeholder_2] => 11 [:db_insert_placeholder_3] => shipping [:db_insert_placeholder_4] => 0 [:db_insert_placeholder_5] => und [:db_insert_placeholder_6] => [:db_insert_placeholder_7] => EUR [:db_insert_placeholder_8] => a:1:{s:10:"components";a:1:{i:0;a:3:{s:4:"name";s:8:"shipping";s:5:"price";a:3:{s:6:"amount";b:0;s:13:"currency_code";s:3:"EUR";s:4:"data";a:0:{}}s:8:"included";b:1;}}} ) in field_sql_storage_field_storage_write() (line 448 of /Users/garethhall/Sites/wearnewzealand.co.nz/public_html_3/modules/field/modules/field_sql_storage/field_sql_storage.module)