diff --git a/sources/user_problem.F90 b/sources/user_problem.F90
index 0f612cb..c2233cf 100644
--- a/sources/user_problem.F90
+++ b/sources/user_problem.F90
@@ -289,7 +289,7 @@ module user_problem
       do i = 1, nn
         bx = bamp * csx(i) * (csy(j) * thy(j) + sny(j) / (pi2 * dlta * ch2(j)))
         by = bamp * 5.0d-01 * snx(i) * sny(j) * thy(j)
-        bz = sqrt(bgui**2 + zeta**2 * max(0.0d+00, bamp**2 - bx**2 - by**2))
+        bz = sqrt(max(0.0d+00, bgui**2 + zeta * (bamp**2 - bx**2 - by**2)))
 
         pdata%q(ibx,i,j,:) = bx
         pdata%q(iby,i,j,:) = by