diff --git a/oser_backend/settings/production.py b/oser_backend/settings/production.py
index f7a85fa5ef8d19b8838d9dd530ea1b29b36db2a9..1211646db84de6be968c5e27340ffc1206fc3c81 100644
--- a/oser_backend/settings/production.py
+++ b/oser_backend/settings/production.py
@@ -6,7 +6,12 @@ from aws.conf import *
 
 from .common import *
 
-DEBUG = os.environ.get('DEBUG', False)
+# NOTE: `or False` ensures the value is  `False` (the boolean)
+# if the value given in environment is false-y (e.g. empty string)
+# Otherwise may lead to unexpected bugs.
+# For example, SendGrid could send an empty string as the sandbox mode,
+# leading to strange 400 Bad Request errors.
+DEBUG = os.environ.get('DEBUG', False) or False
 
 ALLOWED_HOSTS = [
     'localhost',
diff --git a/visits/admin.py b/visits/admin.py
index 3e985031be01c689e9d2e718df9e5a412097e85e..4e1d0a2323c7ac0d2ff0fb9fb511b843be199675 100644
--- a/visits/admin.py
+++ b/visits/admin.py
@@ -17,7 +17,7 @@ class RegistrationsOpenFilter(admin.SimpleListFilter):
     https://docs.djangoproject.com/fr/2.0/ref/contrib/admin/#django.contrib.admin.ModelAdmin.list_filter
     """
 
-    title = "état des inscriptions"
+    title = 'état des inscriptions'
     parameter_name = 'registrations_open'
 
     def lookups(self, request, model_admin):
@@ -48,23 +48,27 @@ class VisitForm(forms.ModelForm):
 
         - Deadline must be before the date
         - End time must be after start time
+
+        Keep in mind that values may be `None` if not provided in the form.
         """
         cleaned_data = super().clean()
         date = cleaned_data.get('date')
         deadline = cleaned_data.get('deadline')
         start_time = cleaned_data.get('start_time')
         end_time = cleaned_data.get('end_time')
-        if deadline.date() >= date:
-            error = forms.ValidationError(
-                "La date limite d'inscription doit être avant la "
-                "date de la sortie."
-            )
-            self.add_error('deadline', error)
-        if end_time <= start_time:
-            error = forms.ValidationError(
-                "L'heure de fin doit être après l'heure de début.")
-            self.add_error('start_time', error)
-            self.add_error('end_time', error)
+        if deadline is not None:
+            if deadline.date() >= date:
+                error = forms.ValidationError(
+                    "La date limite d'inscription doit être avant la "
+                    "date de la sortie."
+                )
+                self.add_error('deadline', error)
+        if end_time is not None and start_time is not None:
+            if end_time <= start_time:
+                error = forms.ValidationError(
+                    "L'heure de fin doit être après l'heure de début.")
+                self.add_error('start_time', error)
+                self.add_error('end_time', error)
 
 
 class ParticipationInline(admin.StackedInline):
diff --git a/visits/models.py b/visits/models.py
index 63fadd8684e1024d63dc718407bcc9f1d2df22e1..31944309f7d678cdb6268db0838c68a559e6126c 100644
--- a/visits/models.py
+++ b/visits/models.py
@@ -216,7 +216,7 @@ class Visit(models.Model):
 
     def get_site_url(self):
         site = Site.objects.get_current()
-        return f'https://{site.domain}/visits/{self.pk}'
+        return f'https://{site.domain}/membres/sorties/{self.pk}'
 
     def __str__(self):
         return str(self.title)