Assessment of tax liability for non-filers permits best judgement assessment and conditional withdrawal upon later return filing. Where a registered person fails to furnish a return after service of notice, the proper officer may assess tax liability to the best of his judgment using available material and issue an assessment order within the statutory limitation from the annual return filing date; if the person files a valid return within a short period after the assessment order, the assessment is deemed withdrawn but interest and late fee liabilities continue.