diff --git a/smash/web/forms.py b/smash/web/forms.py
index 0740fd9bc48810ee9b52b17d2a569e4ab01b09e3..18f662b955b6fed1a4b19b35ffe3a63d0bd2b0b3 100644
--- a/smash/web/forms.py
+++ b/smash/web/forms.py
@@ -84,7 +84,7 @@ class SubjectAddForm(ModelForm):
     def get_prefix_screening_number(self):
         default_location = self.cleaned_data.get('default_location', None)
         screening_number_prefix = None
-        if default_location is not None and default_location.prefix is not None:
+        if default_location is not None and default_location.prefix:
             screening_number_prefix = default_location.prefix
         else:
             subject_type = self.cleaned_data.get('type', None)
diff --git a/smash/web/migrations/0035_screening_number.py b/smash/web/migrations/0035_screening_number.py
index 406772337549cb70cef84e75deb7bcde19eff850..3bf1a4e18c22356fc3a472716073dd1e28f72480 100644
--- a/smash/web/migrations/0035_screening_number.py
+++ b/smash/web/migrations/0035_screening_number.py
@@ -32,7 +32,7 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='location',
             name='prefix',
-            field=models.CharField(max_length=1, null=True),
+            field=models.CharField(max_length=1, blank=True),
         ),
         migrations.RunPython(add_prefixes, reverse_code=lambda x, y: None),
 
diff --git a/smash/web/models/location.py b/smash/web/models/location.py
index 1ff5bbbfc8073cbb9244cbc34c0311fae64f03a3..5720f26897115e0148138e130988a8b2cbfd9181 100644
--- a/smash/web/models/location.py
+++ b/smash/web/models/location.py
@@ -13,7 +13,7 @@ class Location(models.Model):
                              blank=True,
                              default="")
 
-    prefix = models.CharField(max_length=1, null=True)
+    prefix = models.CharField(max_length=1, blank=True)
 
     def __str__(self):
         return "%s" % self.name