diff --git a/aggify/aggify.py b/aggify/aggify.py
index be28cab..a5cca90 100644
--- a/aggify/aggify.py
+++ b/aggify/aggify.py
@@ -576,8 +576,6 @@ def lookup(
                 expected_list=[["local_field", "foreign_field"], "let"]
             )
         elif not let:
-            if not (local_field and foreign_field):
-                raise InvalidArgument(expected_list=["local_field", "foreign_field"])
             lookup_stage = {
                 "$lookup": {
                     "from": from_collection_name,
diff --git a/aggify/utilty.py b/aggify/utilty.py
index 3b40994..ea056ce 100644
--- a/aggify/utilty.py
+++ b/aggify/utilty.py
@@ -6,17 +6,6 @@
 from aggify.types import CollectionType
 
 
-def int_to_slice(final_index: int) -> slice:
-    """
-    Converts an integer to a slice, assuming that the start index is 0.
-
-    Examples:
-        >>> int_to_slice(3)
-        slice(0, 2)
-    """
-    return slice(0, final_index)
-
-
 def to_mongo_positive_index(index: Union[int, slice]) -> slice:
     if isinstance(index, int):
         if index < 0:
diff --git a/tests/test_aggify.py b/tests/test_aggify.py
index 57c5734..b894922 100644
--- a/tests/test_aggify.py
+++ b/tests/test_aggify.py
@@ -11,6 +11,7 @@
     InvalidOperator,
     AlreadyExistsField,
     InvalidEmbeddedField,
+    MongoIndexError,
 )
 
 
@@ -597,3 +598,19 @@ def test_replace_base_invalid_embedded_field(self):
         aggify = Aggify(BaseModel)
         with pytest.raises(InvalidEmbeddedField):
             aggify._replace_base("name")
+
+    def test_aggify_get_item_negative_index(self):
+        with pytest.raises(MongoIndexError):
+            var = Aggify(BaseModel).filter(name=1)[-1:1]
+
+    def test_aggify_get_item_slice_step_not_none(self):
+        with pytest.raises(MongoIndexError):
+            var = Aggify(BaseModel).filter(name=1)[slice(1, 3, 2)]
+
+    def test_aggify_get_item_slice_start_gte_stop(self):
+        with pytest.raises(MongoIndexError):
+            var = Aggify(BaseModel).filter(name=1)[slice(3, 1)]
+
+    def test_aggify_get_item_slice_negative_start(self):
+        with pytest.raises(MongoIndexError):
+            var = Aggify(BaseModel).filter(name=1)[slice(-5, -1)]