diff --git a/src/db/db/dbHierProcessor.cc b/src/db/db/dbHierProcessor.cc
index e65f14a3a..60be2d0f9 100644
--- a/src/db/db/dbHierProcessor.cc
+++ b/src/db/db/dbHierProcessor.cc
@@ -1239,7 +1239,8 @@ private:
   void
   collect_instance_interactions (const db::CellInstArray *inst1, const db::CellInstArray *inst2)
   {
-    //  TODO: this algorithm is not in particular effective for identical arrays
+    //  TODO: this algorithm is not in particular effective for identical arrays or for arrays
+    //  vs. single instances
 
     const db::Cell &cell1 = mp_subject_layout->cell (inst1->object ().cell_index ());
     const db::Cell &cell2 = mp_intruder_layout->cell (inst2->object ().cell_index ());
diff --git a/src/db/unit_tests/dbHierProcessorTests.cc b/src/db/unit_tests/dbHierProcessorTests.cc
index 7e9fe165f..504f36c2e 100644
--- a/src/db/unit_tests/dbHierProcessorTests.cc
+++ b/src/db/unit_tests/dbHierProcessorTests.cc
@@ -1278,3 +1278,9 @@ TEST(FlatOperation)
   run_test_bool22_flat (_this, "hlp17_flat.oas", TMAndNot, 100, 101);
 }
 
+TEST(Arrays)
+{
+  //  Large arrays, NOT
+  run_test_bool2 (_this, "hlp18.oas", TMNot, 100);
+}
+
diff --git a/testdata/algo/hlp18.oas b/testdata/algo/hlp18.oas
new file mode 100644
index 000000000..970739b64
Binary files /dev/null and b/testdata/algo/hlp18.oas differ