Assessee filed original return of income under section 139(4) on 30-3-2013 for the assessment year 2012-13, but failed to claim certain exemption. Assessee filed revised return under section 139(5) for making such claims. Principal Commissioner invoked section 263 and held that assessee was not entitled to file revised return of income since original return was filed under section 139(4).  Tribunal held that that return filed under section 139(4) was validly filed and it was incumbent upon Assessing Officer to complete assessment in pursuance of valid return. However since substitution of sub-section (5) came into force on 1-4-2017 vide Finance Act, 2016 and was prospective in nature, same would not be applicable in instant case. Accordingly assessment made by Assessing Officer was not invalid and non-est but was erroneous and prejudicial to interest of revenue. Revision is held to be valid. (AY. 2012-13)