diff --git a/pyodmongo/services/aggregate_stages.py b/pyodmongo/services/aggregate_stages.py
index 3c71a29..f3ffc0c 100644
--- a/pyodmongo/services/aggregate_stages.py
+++ b/pyodmongo/services/aggregate_stages.py
@@ -134,7 +134,8 @@ def group_set_replace_root(
                 f"_document.{path_str}": {
                     "$cond": {
                         "if": {"$eq": [f"$_document.{array_index}", None]},
-                        "then": None,
+                        "then": [],
+                        # "then": None,
                         "else": f"${field}",
                     }
                 }
diff --git a/pyodmongo/services/reference_pipeline.py b/pyodmongo/services/reference_pipeline.py
index 2fc798d..557ab79 100644
--- a/pyodmongo/services/reference_pipeline.py
+++ b/pyodmongo/services/reference_pipeline.py
@@ -103,12 +103,13 @@ def resolve_reference_pipeline(
             id_ = [
                 f"${e}" for e in unwind_index_list[: len(unwind_index_list) - index - 1]
             ]
-            index_to_unset.append(unwind_index_list[-(index + 1)])
+            current_index = unwind_index_list[-(index + 1)]
+            index_to_unset.append(current_index)
             to_sort = {key: 1 for key in unwind_index_list[1:]}
             pipeline += group_set_replace_root(
                 to_sort=to_sort,
                 id_=id_,
-                array_index=unwind_index_list[-1],
+                array_index=current_index,
                 field=path_str.split(".")[-1],
                 path_str=path_str,
             )
diff --git a/tests/test_engines.py b/tests/test_engines.py
index 8fb6146..78dd233 100644
--- a/tests/test_engines.py
+++ b/tests/test_engines.py
@@ -263,7 +263,7 @@ class B(MainBaseModel):
 
     class C(DbModel):
         name: str = "name_1"
-        b_list: list[B] | None = None
+        b_list: list[B] = []
         _collection: ClassVar = "c"
 
     obj_c = C()