diff --git a/finance/models.py b/finance/models.py
index 47ac532..fbe0ecf 100755
--- a/finance/models.py
+++ b/finance/models.py
@@ -28,7 +28,7 @@ class Bill(models.Model):
         return '%s: %s' % (self.id, self.user)
 
     def freeze_course(self, user):
-        if self.invoice_set.exclude('F').exists():
+        if self.invoice_set.exclude(status='F').exists():
             try:
                 p = Progress.objects.get(user=user, course_token=self.course_token)
                 p.freeze = True