diff --git a/resource_work_time_from_contracts/models/resource_calendar_leaves.py b/resource_work_time_from_contracts/models/resource_calendar_leaves.py
index a2483761b..25437cb17 100644
--- a/resource_work_time_from_contracts/models/resource_calendar_leaves.py
+++ b/resource_work_time_from_contracts/models/resource_calendar_leaves.py
@@ -13,6 +13,9 @@ class ResourceCalendarLeaves(models.Model):
     # leaves for all resources are defined in the same resource calendar,
     # which is needed to compute working hours while taking leaves into
     # account.
+    # fixme A readonly related field without an inverse method should not have a
+    #      default value, as it does not make sense.
+    #      cf odoo/odoo/fields.py:626
     calendar_id = fields.Many2one(
         "resource.calendar",
         related="resource_id.calendar_id",
diff --git a/resource_work_time_from_contracts/models/resource_mixin.py b/resource_work_time_from_contracts/models/resource_mixin.py
index 442869f61..e2daf482b 100644
--- a/resource_work_time_from_contracts/models/resource_mixin.py
+++ b/resource_work_time_from_contracts/models/resource_mixin.py
@@ -6,7 +6,7 @@
 
 from pytz import timezone, utc
 
-from odoo import fields, models
+from odoo import models
 from odoo.tools import float_utils
 
 from odoo.addons.resource.models.resource import ROUNDING_FACTOR
@@ -16,9 +16,6 @@ class ResourceMixin(models.AbstractModel):
 
     _inherit = "resource.mixin"
 
-    # make this field read-only.
-    resource_calendar_id = fields.Many2one("resource.calendar", readonly=True)
-
     def list_work_time_per_day(
         self,
         from_datetime,
diff --git a/resource_work_time_from_contracts/models/resource_resource.py b/resource_work_time_from_contracts/models/resource_resource.py
index 134f6934a..6870149ec 100644
--- a/resource_work_time_from_contracts/models/resource_resource.py
+++ b/resource_work_time_from_contracts/models/resource_resource.py
@@ -9,6 +9,10 @@ class ResourceResource(models.Model):
     _inherit = "resource.resource"
 
     # force this field to be equal to the resource_calendar_id of the company.
+    # fixme A readonly related field without an inverse method should not have a
+    #      default value, as it does not make sense.
+    #      cf odoo/odoo/fields.py:626
+
     calendar_id = fields.Many2one(
         "resource.calendar",
         related="company_id.resource_calendar_id",