diff --git a/src/framework/SurfaceFinder.cc b/src/framework/SurfaceFinder.cc
index 98a5a108..76e76650 100644
--- a/src/framework/SurfaceFinder.cc
+++ b/src/framework/SurfaceFinder.cc
@@ -505,9 +505,9 @@ SurfaceCellInfo SurfaceFinder::PrepareASurfaceCell(
   temp_cell.mu_Q = fluid_cell.mu_C;
   temp_cell.mu_S = fluid_cell.mu_S;
 
-  double u0 =
-      sqrt(1. + fluid_cell.vx * fluid_cell.vx + fluid_cell.vy * fluid_cell.vy +
-           fluid_cell.vz * fluid_cell.vz);
+  double v2 = (fluid_cell.vx * fluid_cell.vx + fluid_cell.vy * fluid_cell.vy
+               + fluid_cell.vz * fluid_cell.vz);
+  double u0 = 1. / sqrt(1. - v2);
   double uz = u0 * fluid_cell.vz;
   temp_cell.umu[0] = u0 * cosh(eta) - uz * sinh(eta);
   temp_cell.umu[1] = u0 * fluid_cell.vx;