diff --git a/InvenTree/order/migrations/0056_alter_salesorderallocation_shipment.py b/InvenTree/order/migrations/0056_alter_salesorderallocation_shipment.py
new file mode 100644
index 0000000000..7a2c255be3
--- /dev/null
+++ b/InvenTree/order/migrations/0056_alter_salesorderallocation_shipment.py
@@ -0,0 +1,19 @@
+# Generated by Django 3.2.5 on 2021-10-25 11:40
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('order', '0055_auto_20211025_0645'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='salesorderallocation',
+            name='shipment',
+            field=models.ForeignKey(help_text='Sales order shipment reference', on_delete=django.db.models.deletion.CASCADE, to='order.salesordershipment', verbose_name='Shipment'),
+        ),
+    ]
diff --git a/InvenTree/order/models.py b/InvenTree/order/models.py
index dcb63430a3..7277897488 100644
--- a/InvenTree/order/models.py
+++ b/InvenTree/order/models.py
@@ -1039,7 +1039,6 @@ class SalesOrderAllocation(models.Model):
     shipment = models.ForeignKey(
         SalesOrderShipment,
         on_delete=models.CASCADE,
-        null=True, blank=True,
         verbose_name=_('Shipment'),
         help_text=_('Sales order shipment reference'),
     )
diff --git a/InvenTree/order/test_migrations.py b/InvenTree/order/test_migrations.py
index 6e4d6668d3..d62449613e 100644
--- a/InvenTree/order/test_migrations.py
+++ b/InvenTree/order/test_migrations.py
@@ -27,10 +27,12 @@ class TestForwardMigrations(MigratorTestCase):
         supplier = Company.objects.create(
             name='Supplier A',
             description='A great supplier!',
-            is_supplier=True
+            is_supplier=True,
+            is_customer=True,
         )
 
         PurchaseOrder = self.old_state.apps.get_model('order', 'purchaseorder')
+        SalesOrder = self.old_state.apps.get_model('order', 'salesorder')
 
         # Create some orders
         for ii in range(10):
@@ -45,19 +47,32 @@ class TestForwardMigrations(MigratorTestCase):
             with self.assertRaises(AttributeError):
                 print(order.reference_int)
 
+            sales_order = SalesOrder.objects.create(
+                customer=supplier,
+                reference=f"{ii}-xyz",
+                description="A test sales order",
+            )
+
+            # Initially, the 'reference_int' field is unavailable
+            with self.assertRaises(AttributeError):
+                print(sales_order.reference_int)
+
     def test_ref_field(self):
         """
         Test that the 'reference_int' field has been created and is filled out correctly
         """
 
         PurchaseOrder = self.new_state.apps.get_model('order', 'purchaseorder')
+        SalesOrder = self.new_state.apps.get_model('order', 'salesorder')
 
         for ii in range(10):
 
-            order = PurchaseOrder.objects.get(reference=f"{ii}-abcde")
+            po = PurchaseOrder.objects.get(reference=f"{ii}-abcde")
+            so = SalesOrder.objects.get(reference=f"{ii}-xyz")
 
             # The integer reference field must have been correctly updated
-        self.assertEqual(order.reference_int, ii)
+            self.assertEqual(po.reference_int, ii)
+            self.assertEqual(so.reference_int, ii)
 
 
 class TestShipmentMigration(MigratorTestCase):
@@ -93,6 +108,10 @@ class TestShipmentMigration(MigratorTestCase):
 
         order.save()
 
+        # The "shipment" model does not exist yet
+        with self.assertRaises(LookupError):
+            self.old_state.apps.get_model('order', 'salesordershipment')
+    
     def test_shipment_creation(self):
         """
         Check that a SalesOrderShipment has been created