diff --git a/src/Singletop/boostx.f b/src/Singletop/boostx.f
index 5ef31a5..23edc75 100644
--- a/src/Singletop/boostx.f
+++ b/src/Singletop/boostx.f
@@ -16,10 +16,9 @@ c--- transformation as required to boost massive vector pt to ptt
       mass=dotpr(pt,pt)
       if (mass < 0._dp) then
         write(6,*) 'mass**2 < 0 in boostx.f, mass**2=',mass
+        mass = abs(mass)
+        call backtrace
         !stop
-        mass = 0._dp
-      else
-          mass=sqrt(mass)
       endif
       mass=sqrt(mass)