diff --git a/src/matcha/subdomain_s.f90 b/src/matcha/subdomain_s.f90
index 5cebb3d..c7047bb 100644
--- a/src/matcha/subdomain_s.f90
+++ b/src/matcha/subdomain_s.f90
@@ -71,9 +71,9 @@
 
   module procedure step
 
-    call assert(allocated(self%s_), "subdomain_t%laplacian: allocated(rhs%s_)")
-    call assert(my_internal_west+1<=my_nx,"laplacian: westernmost subdomain too small")
-    call assert(my_internal_east-1>0,"laplacian: easternmost subdomain too small")
+    call assert(allocated(self%s_), "subdomain_s(step): allocated(rhs%s_)")
+    call assert(my_internal_west+1<=my_nx,"subdomain_s(step): westernmost subdomain too small")
+    call assert(my_internal_east-1>0,"subdomain_s(step): easternmost subdomain too small")
 
     if (.not. allocated(increment)) allocate(increment(my_nx,ny,nz))
  
@@ -175,8 +175,8 @@ pure module function laplacian(rhs) result(laplacian_rhs)
                                 (rhs%s_(i  ,j  ,k-1) - 2*rhs%s_(i,j,k) + rhs%s_(i  ,j  ,k+1))/dz_**2
     end do
 
-    if (me==1) then
-      halo_east = rhs%s_(1,:,:)
+    if (me==num_subdomains) then
+      halo_east = rhs%s_(my_nx,:,:)
     else
       halo_east = rhs[me+1]%s_(lbound(rhs[me+1]%s_,1),:,:)
     end if
@@ -188,10 +188,10 @@ pure module function laplacian(rhs) result(laplacian_rhs)
                                 (rhs%s_(i  ,j  ,k-1) - 2*rhs%s_(i,j,k) + rhs%s_(i  ,j  ,k+1))/dz_**2
     end do
 
-    laplacian_rhs%s_(:, 1,:) = 0.
-    laplacian_rhs%s_(:,ny,:) = 0.
-    laplacian_rhs%s_(:,:, 1) = 0.
-    laplacian_rhs%s_(:,:,nz) = 0.
+    laplacian_rhs%s_(:, 1, :) = 0.
+    laplacian_rhs%s_(:,ny, :) = 0.
+    laplacian_rhs%s_(:, :, 1) = 0.
+    laplacian_rhs%s_(:, :,nz) = 0.
     if (me==1) laplacian_rhs%s_(1,:,:) = 0.
     if (me==num_subdomains) laplacian_rhs%s_(my_nx,:,:) = 0.
 
diff --git a/test/subdomain_test_m.f90 b/test/subdomain_test_m.f90
index 64c8ef9..237e2e6 100644
--- a/test/subdomain_test_m.f90
+++ b/test/subdomain_test_m.f90
@@ -214,7 +214,7 @@ function T_procedural()
 
       call T%define(side, boundary_val, internal_val, n)
 
-      associate(dt => T%dx()*T%dy()/(4*alpha))
+      associate(dt => T%dx()*T%dy()*T%dz()/(4*alpha))
         do step = 1, steps
           sync all
           call step(alpha*dt, T)