diff --git a/doc/getting_started/plugins.rst b/doc/getting_started/plugins.rst
index 7307a2216..ccfdd7374 100644
--- a/doc/getting_started/plugins.rst
+++ b/doc/getting_started/plugins.rst
@@ -43,6 +43,11 @@ This plugin provides a simple management frontend for `amavisd-new
 * Per domain customization (using policies): specify how amavisd-new
   will handle traffic
 
+.. note::
+   The per-domain policies feature only works for new
+   installations. Currently, you can't use modoboa with an existing
+   database (ie. with data in *users* and *policies* tables).
+
 .. note::
 
    This plugin requires *amavisd-new* version **2.7.0** or higher. If
diff --git a/modoboa/extensions/amavis/models.py b/modoboa/extensions/amavis/models.py
index d646fbffb..bd50dafd6 100644
--- a/modoboa/extensions/amavis/models.py
+++ b/modoboa/extensions/amavis/models.py
@@ -132,7 +132,7 @@ class Policy(models.Model):
     spam_admin = models.CharField(max_length=192, blank=True, null=True)
     spam_subject_tag = models.CharField(max_length=192, blank=True, null=True)
     spam_subject_tag2 = models.CharField(
-        ugettext_lazy("Spam marker"), default='',
+        ugettext_lazy("Spam marker"), default=None,
         max_length=192, blank=True, null=True,
         help_text=ugettext_lazy("Modify spam subject using the specified text. Choose 'default' to use global settings.")
         )