Defective return rules: AO may require rectification within prescribed time; failure renders the return invalid, condonation possible. Section 139(9) permits the Assessing Officer to intimate defects in a filed return and allow the assessee a prescribed period, extendable on application, to rectify them; failure to rectify within that period renders the return invalid as if no return were filed, though the AO may condone delay and validate a rectified return if corrected before completion of assessment. Time runs from service of the communication.