diff --git a/jsonpatch.py b/jsonpatch.py
index d3fc26d..df6e3c1 100644
--- a/jsonpatch.py
+++ b/jsonpatch.py
@@ -797,7 +797,8 @@ def _item_added(self, path, key, item):
         index = self.take_index(item, _ST_REMOVE)
         if index is not None:
             op = index[2]
-            if type(op.key) == int and type(key) == int:
+            parent_collection = op.pointer.to_last(self.dst_doc)[0]
+            if isinstance(parent_collection, MutableSequence):
                 for v in self.iter_from(index):
                     op.key = v._on_undo_remove(op.path, op.key)
 
@@ -827,12 +828,8 @@ def _item_removed(self, path, key, item):
         new_index = self.insert(new_op)
         if index is not None:
             op = index[2]
-            # We can't rely on the op.key type since PatchOperation casts
-            # the .key property to int and this path wrongly ends up being taken
-            # for numeric string dict keys while the intention is to only handle lists.
-            # So we do an explicit check on the item affected by the op instead.
-            added_item = op.pointer.to_last(self.dst_doc)[0]
-            if type(added_item) == list:
+            parent_collection = op.pointer.to_last(self.dst_doc)[0]
+            if isinstance(parent_collection, MutableSequence):
                 for v in self.iter_from(index):
                     op.key = v._on_undo_add(op.path, op.key)
 
diff --git a/tests.py b/tests.py
index d9eea92..32239dc 100755
--- a/tests.py
+++ b/tests.py
@@ -566,6 +566,14 @@ def test_issue120(self):
         res = jsonpatch.apply_patch(src, patch)
         self.assertEqual(res, dst)
 
+    def test_issue_160(self):
+        """Removal of an operation to an array should trigger _on_undo_add."""
+        old = {'a': [{'id': [1]}, {'id': [2]}], 'b': [{'id': 5}]}
+        new = {'a': [{'id': []}, {'id': [1]}], 'b': [{'id': 5, 'newKey': 2}]}
+        patch = jsonpatch.make_patch(old, new)
+        result = jsonpatch.apply_patch(old, patch)
+        self.assertEqual(result, new)
+
     def test_custom_types_diff(self):
         old = {'value': decimal.Decimal('1.0')}
         new = {'value': decimal.Decimal('1.00')}