diff --git a/submissions/migrations/0025_auto_20180520_1430.py b/submissions/migrations/0025_auto_20180520_1430.py
new file mode 100644
index 0000000000000000000000000000000000000000..9ce09ec3932626120e826247e926e6461d5c4748
--- /dev/null
+++ b/submissions/migrations/0025_auto_20180520_1430.py
@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.4 on 2018-05-20 12:30
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('submissions', '0024_auto_20180519_2324'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='refereeinvitation',
+            name='invitation_key',
+            field=models.CharField(blank=True, max_length=40),
+        ),
+    ]
diff --git a/submissions/models.py b/submissions/models.py
index 7730c39bccfbf08d9904a45d7d2139947aa189b1..808f95b7ce8717873efca89aaa198b3955494071 100644
--- a/submissions/models.py
+++ b/submissions/models.py
@@ -409,7 +409,7 @@ class RefereeInvitation(SubmissionRelatedObjectMixin, models.Model):
     email_address = models.EmailField()
 
     # if Contributor not found, person is invited to register
-    invitation_key = models.CharField(max_length=40)
+    invitation_key = models.CharField(max_length=40, blank=True)
     date_invited = models.DateTimeField(default=timezone.now)
     invited_by = models.ForeignKey('scipost.Contributor', related_name='referee_invited_by',
                                    blank=True, null=True, on_delete=models.CASCADE)
diff --git a/submissions/utils.py b/submissions/utils.py
index 987fccf16b9bde6417803f405895d7ed8f7b052a..39d3ee95c163389f3d38983d0f7ae4463c6dca2b 100644
--- a/submissions/utils.py
+++ b/submissions/utils.py
@@ -166,7 +166,7 @@ class BaseRefereeSubmissionCycle(BaseSubmissionCycle):
         for ref_inv in self.submission.referee_invitations.all():
             if not ref_inv.cancelled:
                 if ref_inv.accepted is None:
-                    '''An invited referee may have not responded yet.'''
+                    # An invited referee may have not responded yet.
                     timelapse = timezone.now() - ref_inv.date_invited
                     if timelapse > datetime.timedelta(days=3):
                         text = ('Referee %s has not responded for %i days. '
@@ -174,7 +174,7 @@ class BaseRefereeSubmissionCycle(BaseSubmissionCycle):
                                 % (ref_inv.referee_str, timelapse.days))
                         self.required_actions.append(('referee_no_response', text,))
                 elif ref_inv.accepted and not ref_inv.fulfilled:
-                    '''A referee has not fulfilled its duty and the deadline is closing in.'''
+                    # A referee has not fulfilled its duty and the deadline is closing in.
                     timeleft = self.submission.reporting_deadline - timezone.now()
                     if timeleft < datetime.timedelta(days=7):
                         text = ('Referee %s has accepted to send a Report, '
diff --git a/submissions/views.py b/submissions/views.py
index f1f6021107ff9e7bf8c47299259b104d4194995c..d92b18082a9292f7036d370f8b2184c69ddd2e52 100644
--- a/submissions/views.py
+++ b/submissions/views.py
@@ -879,7 +879,6 @@ def send_refereeing_invitation(request, arxiv_identifier_w_vn_nr, contributor_id
         last_name=contributor.user.last_name,
         email_address=contributor.user.email,
         # the key is only used for inviting unregistered users
-        invitation_key='notused',
         date_invited=timezone.now(),
         invited_by=request.user.contributor)