diff --git a/include/bout/interpolation_z.hxx b/include/bout/interpolation_z.hxx
index 68cf5b0b06..c2ff03ff59 100644
--- a/include/bout/interpolation_z.hxx
+++ b/include/bout/interpolation_z.hxx
@@ -35,7 +35,7 @@ protected:
 
 public:
   explicit ZInterpolation(int y_offset = 0, Mesh* mesh = nullptr,
-                          Region<Ind3D> region_in = {});
+                          const Region<Ind3D>& region_in = {});
   virtual ~ZInterpolation() = default;
 
   virtual void calcWeights(const Field3D& delta_z) = 0;
diff --git a/src/mesh/interpolation/interpolation_z.cxx b/src/mesh/interpolation/interpolation_z.cxx
index 8d39e6baa8..e867eb858c 100644
--- a/src/mesh/interpolation/interpolation_z.cxx
+++ b/src/mesh/interpolation/interpolation_z.cxx
@@ -23,34 +23,7 @@
 #include <bout/interpolation_z.hxx>
 #include <bout/mesh.hxx>
 
-ZInterpolation::ZInterpolation(int y_offset, Mesh* mesh, Region<Ind3D> region_in)
-    : localmesh(mesh == nullptr ? bout::globals::mesh : mesh), region(region_in),
-      y_offset(y_offset) {
-  if (region.size() == 0) {
-    // Construct region that skips calculating interpolation in y-boundary regions that
-    // should be filled by boundary conditions
-
-    region = localmesh->getRegion3D("RGN_NOBNDRY");
-
-    const int ny = localmesh->LocalNy;
-    const int nz = localmesh->LocalNz;
-    auto mask_region = Region<Ind3D>(0, -1, 0, -1, 0, 0, ny, nz);
-    if (y_offset > 0) {
-      for (auto it = localmesh->iterateBndryUpperY(); not it.isDone(); it.next()) {
-        mask_region +=
-            Region<Ind3D>(it.ind, it.ind, localmesh->yend - y_offset + 1, localmesh->yend,
-                          localmesh->zstart, localmesh->zend, ny, nz);
-      }
-    } else if (y_offset < 0) {
-      for (auto it = localmesh->iterateBndryLowerY(); not it.isDone(); it.next()) {
-        mask_region += Region<Ind3D>(it.ind, it.ind, localmesh->ystart,
-                                     localmesh->ystart - y_offset - 1, localmesh->zstart,
-                                     localmesh->zend, ny, nz);
-      }
-    }
-
-    mask_region.unique();
-
-    region.mask(mask_region);
-  }
-}
+ZInterpolation::ZInterpolation(int y_offset, Mesh* mesh, const Region<Ind3D>& region_in)
+    : localmesh(mesh == nullptr ? bout::globals::mesh : mesh),
+      region(region_in.size() == 0 ? localmesh->getRegion3D("RGN_NOY") : region_in),
+      y_offset(y_offset) {}
diff --git a/tests/unit/fake_mesh.hxx b/tests/unit/fake_mesh.hxx
index 7d7326c149..4656ee0282 100644
--- a/tests/unit/fake_mesh.hxx
+++ b/tests/unit/fake_mesh.hxx
@@ -43,6 +43,9 @@ public:
     GlobalNx = nx;
     GlobalNy = ny;
     GlobalNz = nz;
+    GlobalNxNoBoundaries = nx - 2;
+    GlobalNyNoBoundaries = ny - 2;
+    GlobalNzNoBoundaries = nz;
     LocalNx = nx;
     LocalNy = ny;
     LocalNz = nz;
diff --git a/tests/unit/mesh/parallel/test_shiftedmetric.cxx b/tests/unit/mesh/parallel/test_shiftedmetric.cxx
index 579e6fdd7b..97775133e5 100644
--- a/tests/unit/mesh/parallel/test_shiftedmetric.cxx
+++ b/tests/unit/mesh/parallel/test_shiftedmetric.cxx
@@ -314,35 +314,19 @@ TYPED_TEST(ShiftedMetricTest, CalcParallelSlices) {
   // We don't shift in the guard cells, and the parallel slices are
   // stored offset in y, therefore we need to make new regions that we
   // can compare the expected and actual outputs over
-
-  constexpr bool is_shifted_interp = std::is_same_v<TypeParam, ShiftedMetricInterp>;
-
-  // ShiftedMetricInterp doesn't seem to store interpolated values in _any_ of the
-  // guards, so we can only check the interior X point
-  const int xstart = is_shifted_interp ? mesh->xstart : 0;
-  const int xend = is_shifted_interp ? mesh->xend : mesh->LocalNx - 1;
-
-  // It also means we can't check in the _y_ guards either.
-  // TODO(peter): Is this a bug?
-  const int yup_1_end = is_shifted_interp ? mesh->yend : mesh->yend + 1;
-  const int yup_2_end = is_shifted_interp ? mesh->yend : mesh->yend + 2;
-
-  const int ydown_1_start = is_shifted_interp ? mesh->ystart : mesh->ystart - 1;
-  const int ydown_2_start = is_shifted_interp ? mesh->ystart : mesh->ystart - 2;
-
   mesh->addRegion3D("RGN_YUP",
-                    Region<Ind3D>(xstart, xend, mesh->ystart + 1, yup_1_end, 0,
-                                  mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
+                    Region<Ind3D>(0, mesh->LocalNx - 1, mesh->ystart + 1, mesh->yend + 1,
+                                  0, mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
   mesh->addRegion3D("RGN_YUP2",
-                    Region<Ind3D>(xstart, xend, mesh->ystart + 2, yup_2_end, 0,
-                                  mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
+                    Region<Ind3D>(0, mesh->LocalNx - 1, mesh->ystart + 2, mesh->yend + 2,
+                                  0, mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
 
   mesh->addRegion3D("RGN_YDOWN",
-                    Region<Ind3D>(xstart, xend, ydown_1_start, mesh->yend - 1, 0,
-                                  mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
+                    Region<Ind3D>(0, mesh->LocalNx - 1, mesh->ystart - 1, mesh->yend - 1,
+                                  0, mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
   mesh->addRegion3D("RGN_YDOWN2",
-                    Region<Ind3D>(xstart, xend, ydown_2_start, mesh->yend - 2, 0,
-                                  mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
+                    Region<Ind3D>(0, mesh->LocalNx - 1, mesh->ystart - 2, mesh->yend - 2,
+                                  0, mesh->LocalNz - 1, mesh->LocalNy, mesh->LocalNz));
   output_info.enable();
 
   // Actual interesting bit here!