diff --git a/wagtail/core/migrations/0047_add_workflow_models.py b/wagtail/core/migrations/0047_add_workflow_models.py
index 4994363eba..84624563ca 100755
--- a/wagtail/core/migrations/0047_add_workflow_models.py
+++ b/wagtail/core/migrations/0047_add_workflow_models.py
@@ -1,4 +1,4 @@
-# Generated by Django 3.0.3 on 2020-02-17 10:19
+# Generated by Django 3.0.3 on 2020-02-18 15:33
 
 from django.conf import settings
 from django.db import migrations, models
@@ -10,9 +10,9 @@ import wagtail.core.models
 class Migration(migrations.Migration):
 
     dependencies = [
-        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
         ('contenttypes', '0002_remove_content_type_name'),
         ('wagtailcore', '0046_site_name_remove_null'),
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
         ('auth', '0011_update_proxy_permissions'),
     ]
 
@@ -76,7 +76,7 @@ class Migration(migrations.Migration):
                 ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
                 ('status', models.CharField(choices=[('in_progress', 'In progress'), ('approved', 'Approved'), ('rejected', 'Rejected'), ('cancelled', 'Cancelled')], default='in_progress', max_length=50, verbose_name='status')),
                 ('created_at', models.DateTimeField(auto_now_add=True, verbose_name='created at')),
-                ('current_task_state', models.OneToOneField(null=True, on_delete=django.db.models.deletion.SET_NULL, to='wagtailcore.TaskState', verbose_name='current task state')),
+                ('current_task_state', models.OneToOneField(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, to='wagtailcore.TaskState', verbose_name='current task state')),
                 ('page', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='workflow_states', to='wagtailcore.Page', verbose_name='page')),
                 ('requested_by', models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='requested_workflows', to=settings.AUTH_USER_MODEL, verbose_name='requested by')),
                 ('workflow', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='workflow_states', to='wagtailcore.Workflow', verbose_name='workflow')),
diff --git a/wagtail/core/models.py b/wagtail/core/models.py
index 665dcc54e3..9926d62c6d 100644
--- a/wagtail/core/models.py
+++ b/wagtail/core/models.py
@@ -2785,12 +2785,22 @@ class WorkflowState(models.Model):
                                      editable=True,
                                      on_delete=models.SET_NULL,
                                      related_name='requested_workflows')
-    current_task_state = models.OneToOneField('TaskState', on_delete=models.SET_NULL, null=True, blank=False,
+    current_task_state = models.OneToOneField('TaskState', on_delete=models.SET_NULL, null=True, blank=True,
                                               verbose_name=_("current task state"))
 
     # allows a custom function to be called on finishing the Workflow successfully.
     on_finish = import_string(getattr(settings, 'WAGTAIL_FINISH_WORKFLOW_ACTION', 'wagtail.core.workflows.publish_workflow_state'))
 
+    def clean(self):
+        super().clean()
+        # The unique constraint is conditional, and so not supported on the MySQL backend - so an additional check is done here
+        if WorkflowState.objects.filter(status=self.STATUS_IN_PROGRESS, page=self.page).exclude(pk=self.pk).exists():
+            raise ValidationError(_('There may only be one in progress workflow state per page.'))
+
+    def save(self, *args, **kwargs):
+        self.full_clean()
+        return super().save(*args, **kwargs)
+
     def __str__(self):
         return _("Workflow '{0}' on Page '{1}': {2}").format(self.workflow, self.page, self.status)
 
@@ -2884,7 +2894,7 @@ class WorkflowState(models.Model):
     class Meta:
         verbose_name = _('Workflow state')
         verbose_name_plural = _('Workflow states')
-        # prevent multiple STATUS_IN_PROGRESS workflows for the same page
+        # prevent multiple STATUS_IN_PROGRESS workflows for the same page. This is not supported by MySQL, so is checked additionally on save.
         constraints = [
             models.UniqueConstraint(fields=['page'], condition=Q(status='in_progress'), name='unique_in_progress_workflow')
         ]
diff --git a/wagtail/core/tests/test_workflow.py b/wagtail/core/tests/test_workflow.py
index 872417c648..ead218e437 100644
--- a/wagtail/core/tests/test_workflow.py
+++ b/wagtail/core/tests/test_workflow.py
@@ -3,6 +3,7 @@ import datetime
 import pytz
 from django.contrib.auth import get_user_model
 from django.contrib.auth.models import Group
+from django.core.exceptions import ValidationError
 from django.db.utils import IntegrityError
 from django.test import TestCase, override_settings
 
@@ -115,7 +116,7 @@ class TestWorkflows(TestCase):
     def test_error_when_starting_multiple_in_progress_workflows(self):
         # test trying to start multiple status='in_progress' workflows on a single page will trigger an IntegrityError
         self.start_workflow_on_homepage()
-        with self.assertRaises(IntegrityError):
+        with self.assertRaises((IntegrityError, ValidationError)):
             self.start_workflow_on_homepage()
 
     @freeze_time("2017-01-01 12:00:00")