diff --git a/openvdb/openvdb/tools/MeshToVolume.h b/openvdb/openvdb/tools/MeshToVolume.h
index 2aac956e75..669ed621d7 100644
--- a/openvdb/openvdb/tools/MeshToVolume.h
+++ b/openvdb/openvdb/tools/MeshToVolume.h
@@ -1645,7 +1645,7 @@ checkNeighbours(const Index pos, const typename LeafNodeType::ValueType * data,
     // i + 1, j, k
     if (mask[0] && Compare::check(data[pos + NodeT::DIM * NodeT::DIM]))                   return true;
     // i+1, j, k-1
-    if (mask[6] && Compare::check(data[pos + NodeT::DIM * NodeT::DIM]))                   return true;
+    if (mask[6] && Compare::check(data[pos + NodeT::DIM * NodeT::DIM - 1]))               return true;
     // i-1, j, k-1
     if (mask[7] && Compare::check(data[pos - NodeT::DIM * NodeT::DIM - 1]))               return true;
     // i+1, j, k+1
diff --git a/openvdb/openvdb/unittest/TestMeshToVolume.cc b/openvdb/openvdb/unittest/TestMeshToVolume.cc
index 5c4ff2ffdf..2042e1ad24 100644
--- a/openvdb/openvdb/unittest/TestMeshToVolume.cc
+++ b/openvdb/openvdb/unittest/TestMeshToVolume.cc
@@ -60,6 +60,39 @@ TEST_F(TestMeshToVolume, testUtils)
     EXPECT_TRUE( mZ == 9);
 }
 
+// Regression test for the checkNeighbours(pos, data, mask) overload in
+// mesh_to_volume_internal: each of its 26 branches hand-computes a flat-array
+// offset for one entry of util::COORD_OFFSETS from the node's i/j/k strides, so a
+// copy-paste slip in any single branch (as happened for i+1,j,k-1, which reused
+// the i+1,j,k offset and so silently ignored a negative neighbour on read) goes
+// undetected by callers that never place a negative value at that exact offset.
+// This checks all 26 branches against the offset table directly.
+TEST_F(TestMeshToVolume, testCheckNeighboursOffsets)
+{
+    using namespace openvdb;
+    using namespace openvdb::tools::mesh_to_volume_internal;
+
+    using LeafT = FloatTree::LeafNodeType;
+    struct IsNegative { static bool check(float v) { return v < 0.0f; } };
+
+    // interior voxel, far enough from every face that all 26 neighbours are in-node
+    const Coord center(LeafT::DIM / 2, LeafT::DIM / 2, LeafT::DIM / 2);
+    const Index pos = LeafT::coordToOffset(center);
+
+    for (int m = 0; m < 26; ++m) {
+        std::vector<float> data(LeafT::NUM_VALUES, /*background=*/1.0f);
+        const Coord neighbourIjk = center + util::COORD_OFFSETS[m];
+        data[LeafT::coordToOffset(neighbourIjk)] = -1.0f;
+
+        bool mask[26] = {false};
+        mask[m] = true;
+
+        EXPECT_TRUE((checkNeighbours<IsNegative, LeafT>(pos, data.data(), mask)))
+            << "checkNeighbours missed neighbour offset " << m
+            << " = " << util::COORD_OFFSETS[m];
+    }
+}
+
 TEST_F(TestMeshToVolume, testConversion)
 {
     using namespace openvdb;
diff --git a/pendingchanges/openvdb_tools.txt b/pendingchanges/openvdb_tools.txt
index 7a6aea662f..df7befe90a 100644
--- a/pendingchanges/openvdb_tools.txt
+++ b/pendingchanges/openvdb_tools.txt
@@ -18,3 +18,4 @@ tools:
     maximum allowed) instead of crashing. The check is confined to process() so that offset() -- used
     by the offset-only path -- continues to accept coarser voxel sizes unaffected.
   - Fixed a floating-point false positive in "tools::CheckLevelSet::checkBackground()" in Diagnostics.h where the ratio "background / voxelSize" could compute to just below the expected half-width due to non-representable IEEE 754 values, causing a valid level set created with exactly LEVEL_SET_HALF_WIDTH voxels of narrow band to incorrectly fail check #4. The strict "<" comparison is now relaxed by a relative tolerance of 1e-6. A regression test covering this exact case was added to TestDiagnostics.cc.
+  - Fixed an index error in "mesh_to_volume_internal::checkNeighbours()" in MeshToVolume.h where the i+1,j,k-1 neighbour reused the i+1,j,k offset, silently missing that neighbour's value during self-intersection checks in mesh-to-volume conversion (bugfix contributed by hexuejun). A regression test covering all 26 neighbour offsets was added to TestMeshToVolume.cc.