diff --git a/src/atlas/interpolation/method/structured/StructuredInterpolation2D.tcc b/src/atlas/interpolation/method/structured/StructuredInterpolation2D.tcc
index 6dc81cc40..f80d23bfa 100644
--- a/src/atlas/interpolation/method/structured/StructuredInterpolation2D.tcc
+++ b/src/atlas/interpolation/method/structured/StructuredInterpolation2D.tcc
@@ -433,9 +433,10 @@ void StructuredInterpolation2D<Kernel>::setup( const FunctionSpace& source ) {
                 auto inp_npts_owned = src_fs.sizeOwned();
 
                 for (auto& t : triplets) {
-                    ATLAS_ASSERT(t.col() < inp_npts && gidx_src(t.col()) >= 1);
-                    t = {t.row(), eckit::linalg::Size(gidx_src(t.col()) - 1), t.value()};
-                    ATLAS_ASSERT(t.col() < inp_npts_owned);
+                    auto& col = t.col();
+                    ATLAS_ASSERT(col < inp_npts && gidx_src(col) >= 1);
+                    col = gidx_src(col) - 1;
+                    ATLAS_ASSERT(col < inp_npts_owned);
                 }
 
                 inp_npts = inp_npts_owned;