The successive versions of the GTAP databases are provided with GTAPAgg, a programme that computes values of the series of the database for any regional and sectoral aggregation. A ‘self-trade’ issue arises from the fact that GTAPAgg aggregates the several series concerned with international trade, like any other series, by simply summing them up: the resulting series include a share of exports and imports happening between the aggregated regions, which should rather be treated as economic flows internal to the region resulting from the aggregation process. This paper details a method that aims at solving this shortcoming, and discusses the importance of doing so. A first section puts the research question into context and discusses its likely importance when using GTAP as a calibration dataset for computable general equilibrium modelling. A second section details the analytics of the method proposed to correct the aggregation process, in the broader framework of a programme extending GTAPAgg to the production of national account matrixes in the standard United Nations format. An appendix provides the code of the extended aggregation programme developed.