diff --git a/doc/changelog.rst b/doc/changelog.rst
index 9a0460f..9adac30 100644
--- a/doc/changelog.rst
+++ b/doc/changelog.rst
@@ -7,6 +7,7 @@ Changelog
 Fixed
 ^^^^^
 - Fix :attr:`~SearchRequest.start_index` and :attr:`~SearchRequest.count` limits. :issue:`84`
+- :attr:`~ListResponse.total_resuls` is required.
 
 [0.3.0] - 2024-12-11
 --------------------
diff --git a/scim2_models/rfc7644/list_response.py b/scim2_models/rfc7644/list_response.py
index 099bee8..af55ec6 100644
--- a/scim2_models/rfc7644/list_response.py
+++ b/scim2_models/rfc7644/list_response.py
@@ -104,7 +104,13 @@ class ListResponse(Message, Generic[AnyResource], metaclass=ListResponseMetaclas
     def check_results_number(
         cls, value: Any, handler: ValidatorFunctionWrapHandler, info: ValidationInfo
     ) -> Self:
-        """:rfc:`RFC7644 §3.4.2 <7644#section-3.4.2.4>` indicates that 'resources' must be set if 'totalResults' is non-zero."""
+        """Validate result numbers.
+
+        :rfc:`RFC7644 §3.4.2 <7644#section-3.4.2.4>` indicates that:
+
+        - 'totalResults' is required
+        - 'resources' must be set if 'totalResults' is non-zero.
+        """
         obj = handler(value)
 
         if (
@@ -114,6 +120,12 @@ def check_results_number(
         ):
             return obj
 
+        if obj.total_results is None:
+            raise PydanticCustomError(
+                "required_error",
+                "Field 'total_results' is required but value is missing or null",
+            )
+
         if obj.total_results > 0 and not obj.resources:
             raise PydanticCustomError(
                 "no_resource_error",
diff --git a/tests/test_list_response.py b/tests/test_list_response.py
index 071cb5d..cf9419e 100644
--- a/tests/test_list_response.py
+++ b/tests/test_list_response.py
@@ -217,3 +217,26 @@ def test_list_response_schema_ordering():
         ],
     }
     ListResponse[Union[User[EnterpriseUser], Group]].model_validate(payload)
+
+
+def test_total_results_required():
+    """ListResponse.total_results is required."""
+    payload = {
+        "Resources": [
+            {
+                "schemas": [
+                    "urn:ietf:params:scim:schemas:core:2.0:User",
+                ],
+                "userName": "bjensen@example.com",
+                "id": "foobar",
+            }
+        ],
+    }
+
+    with pytest.raises(
+        ValidationError,
+        match="Field 'total_results' is required but value is missing or null",
+    ):
+        ListResponse[User].model_validate(
+            payload, scim_ctx=Context.RESOURCE_QUERY_RESPONSE
+        )