diff --git a/grid.cc b/grid.cc
index 44ae9c452..c84b7e606 100644
--- a/grid.cc
+++ b/grid.cc
@@ -2451,12 +2451,12 @@ auto get_totmassradionuclide(const int z, const int a) -> double {
           // packet pos below min
           const double boundaryposmin = grid::get_cellcoordmin(cellindex, d) / globals::tmin * tstart;
           delta = pktposgridcoord[d] - boundaryposmin;
-          isoutside_thisside = pos_component_vel_relative_to_flow && (pktposgridcoord[d] < (boundaryposmin - 10.));
+          isoutside_thisside = !pos_component_vel_relative_to_flow && (pktposgridcoord[d] < (boundaryposmin - 10.));
         } else {
           // packet pos above max
           const double boundaryposmax = cellcoordmax[d] / globals::tmin * tstart;
           delta = pktposgridcoord[d] - boundaryposmax;
-          isoutside_thisside = !pos_component_vel_relative_to_flow && (pktposgridcoord[d] > (boundaryposmax + 10.));
+          isoutside_thisside = pos_component_vel_relative_to_flow && (pktposgridcoord[d] > (boundaryposmax + 10.));
         }
 
         if (isoutside_thisside) {