ERROR ITMS-3000

" Your book could not be delivered to the iBookstore" Got this below error message from Apple. Anyone with any idea where I should start to search for errors? The e-book is in Apple´s "Fixed Layout Format".

ERROR ITMS-3000: "Line 344 column 29: element "data_file" incomplete; missing required element "size" at XPath /package/book/assets/asset[1]data_file"