diff --git a/gemet/thesaurus/migrations/0011_auto_20170628_1931.py b/gemet/thesaurus/migrations/0011_auto_20170628_1931.py
new file mode 100644
index 0000000..15f850f
--- /dev/null
+++ b/gemet/thesaurus/migrations/0011_auto_20170628_1931.py
@@ -0,0 +1,45 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.6 on 2017-06-28 16:31
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('thesaurus', '0010_asynctask'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='definitionsource',
+            name='author',
+            field=models.CharField(blank=True, max_length=255, null=True),
+        ),
+        migrations.AlterField(
+            model_name='definitionsource',
+            name='place',
+            field=models.CharField(blank=True, max_length=255, null=True),
+        ),
+        migrations.AlterField(
+            model_name='definitionsource',
+            name='publication',
+            field=models.CharField(blank=True, max_length=255, null=True),
+        ),
+        migrations.AlterField(
+            model_name='definitionsource',
+            name='title',
+            field=models.CharField(blank=True, max_length=255, null=True),
+        ),
+        migrations.AlterField(
+            model_name='definitionsource',
+            name='url',
+            field=models.CharField(blank=True, max_length=255, null=True),
+        ),
+        migrations.AlterField(
+            model_name='definitionsource',
+            name='year',
+            field=models.CharField(blank=True, max_length=10, null=True),
+        ),
+    ]
diff --git a/gemet/thesaurus/models.py b/gemet/thesaurus/models.py
index f0c5985..4e142ed 100644
--- a/gemet/thesaurus/models.py
+++ b/gemet/thesaurus/models.py
@@ -331,12 +331,12 @@ class ForeignRelation(VersionableModel):
 
 class DefinitionSource(models.Model):
     abbr = models.CharField(max_length=10, primary_key=True)
-    author = models.CharField(max_length=255, null=True)
-    title = models.CharField(max_length=255, null=True)
-    url = models.CharField(max_length=255, null=True)
-    publication = models.CharField(max_length=255, null=True)
-    place = models.CharField(max_length=255, null=True)
-    year = models.CharField(max_length=10, null=True)
+    author = models.CharField(max_length=255, null=True, blank=True)
+    title = models.CharField(max_length=255, null=True, blank=True)
+    url = models.CharField(max_length=255, null=True, blank=True)
+    publication = models.CharField(max_length=255, null=True, blank=True)
+    place = models.CharField(max_length=255, null=True, blank=True)
+    year = models.CharField(max_length=10, null=True, blank=True)
 
 
 class AsyncTask(models.Model):