diff --git a/CHANGELOG.md b/CHANGELOG.md
index 40208a670..f496d2afb 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,7 @@ and this project adheres to
 ### Fixed
 
 - Fix type of `statement.result.score.scaled` from `int` to `Decimal`
+- Fix optional `member` field in Identified Groups not actually being optional
 - Fix misregistered `TypeError` in Pydantic models validation
 - Fix `check-changelog` CI job by replacing the removed `git whatchanged`
   command with `git log --name-only`
diff --git a/src/ralph/models/xapi/base/groups.py b/src/ralph/models/xapi/base/groups.py
index e47874b6a..1f88367f1 100644
--- a/src/ralph/models/xapi/base/groups.py
+++ b/src/ralph/models/xapi/base/groups.py
@@ -56,7 +56,7 @@ class BaseXapiIdentifiedGroup(BaseXapiGroupCommonProperties):
         member (list): Consist of a list of the members of this Group.
     """
 
-    member: Optional[List[BaseXapiAgent]]
+    member: Optional[List[BaseXapiAgent]] = None
 
 
 class BaseXapiIdentifiedGroupWithMbox(BaseXapiIdentifiedGroup, BaseXapiMboxIFI):
diff --git a/tests/models/xapi/base/test_groups.py b/tests/models/xapi/base/test_groups.py
index 9bb41bbff..c6a11dbda 100644
--- a/tests/models/xapi/base/test_groups.py
+++ b/tests/models/xapi/base/test_groups.py
@@ -1,6 +1,13 @@
 """Tests for the base xAPI `Group` definitions."""
 
-from ralph.models.xapi.base.groups import BaseXapiGroupCommonProperties
+import pytest
+from pydantic import ValidationError
+
+from ralph.models.xapi.base.groups import (
+    BaseXapiGroupCommonProperties,
+    BaseXapiIdentifiedGroupWithMbox,
+    BaseXapiIdentifiedGroupWithMboxSha1Sum,
+)
 
 from tests.factories import mock_xapi_instance
 
@@ -12,3 +19,53 @@ def test_models_xapi_base_groups_group_common_properties_with_valid_field():
     field = mock_xapi_instance(BaseXapiGroupCommonProperties)
 
     assert field.objectType == "Group"
+
+
+@pytest.mark.parametrize(
+    "group",
+    [
+        {
+            "name": "d75662f7c72c8d12267712aaf31b649cdab13cb8",
+            "mbox_sha1sum": "6395037c217b04eae94355900bcaf014772c7d7c",
+            "objectType": "Group",
+        },
+        {
+            "name": "d75662f7c72c8d12267712aaf31b649cdab13cb8",
+            "mbox_sha1sum": "6395037c217b04eae94355900bcaf014772c7d7c",
+            "objectType": "Group",
+            "member": [],
+        },
+    ],
+)
+def test_models_xapi_base_group_mbox_sha1sum_valid(group):
+    """Test an invalid group with `mbox_sha1sum` property"""
+
+    try:
+        BaseXapiIdentifiedGroupWithMboxSha1Sum(**group)
+    except ValidationError as err:
+        pytest.fail(f"Valid statement should not raise exceptions: {err}")
+
+
+@pytest.mark.parametrize(
+    "group",
+    [
+        {
+            "name": "d75662f7c72c8d12267712aaf31b649cdab13cb8",
+            "mbox": "mailto:foob@r.com",
+            "objectType": "Group",
+        },
+        {
+            "name": "d75662f7c72c8d12267712aaf31b649cdab13cb8",
+            "mbox": "mailto:foob@r.com",
+            "objectType": "Group",
+            "member": [],
+        },
+    ],
+)
+def test_models_xapi_base_group_mbox_valid(group):
+    """Test an invalid group with `mbox` property"""
+
+    try:
+        BaseXapiIdentifiedGroupWithMbox(**group)
+    except ValidationError as err:
+        pytest.fail(f"Valid statement should not raise exceptions: {err}")