diff --git a/grid.cc b/grid.cc
index d6907600c..b2e18f901 100644
--- a/grid.cc
+++ b/grid.cc
@@ -2548,7 +2548,7 @@ auto get_totmassradionuclide(const int z, const int a) -> double {
                                         tstart;
 
       d_coordmaxboundary[d] = CLIGHT_PROP * t_coordmaxboundary;
-      if (last_cross == negdirections[d]) {
+      if ((pktvelgridcoord[d] - (pktposgridcoord[d] / tstart)) < 0) {
         d_coordmaxboundary[d] = -1;
       }
     }