diff --git a/src/hb-ot-var-gvar-table.hh b/src/hb-ot-var-gvar-table.hh
index 0c3dbd2ee..2b7f5e1c8 100644
--- a/src/hb-ot-var-gvar-table.hh
+++ b/src/hb-ot-var-gvar-table.hh
@@ -761,8 +761,8 @@ struct gvar
 	    if (phantom_only && pt_index < count - 4) continue;
 	    auto &delta = deltas.arrayZ[pt_index];
 	    delta.flag = 1;	/* this point is referenced, i.e., explicit deltas specified */
-	    delta.x += x_deltas.arrayZ[i] * scalar;
-	    delta.y += y_deltas.arrayZ[i] * scalar;
+	    delta.add_delta (x_deltas.arrayZ[i] * scalar,
+			     y_deltas.arrayZ[i] * scalar);
 	  }
 	}
 	else