diff --git a/abilian/web/forms/widgets.py b/abilian/web/forms/widgets.py
index <HASH>..<HASH> 100644
--- a/abilian/web/forms/widgets.py
+++ b/abilian/web/forms/widgets.py
@@ -292,7 +292,8 @@ class AjaxMainTableView(object):
     advanced_search_filters = [dict(name=c.name,
                                     label=unicode(c.label),
                                     type=c.form_filter_type,
-                                    args=c.form_filter_args)
+                                    args=c.form_filter_args,
+                                    unset=c.form_unset_value,)
                                for c in self.search_criterions
                                if c.has_form_filter]
     if advanced_search_filters:
diff --git a/abilian/web/search/criterion.py b/abilian/web/search/criterion.py
index <HASH>..<HASH> 100644
--- a/abilian/web/search/criterion.py
+++ b/abilian/web/search/criterion.py
@@ -47,6 +47,10 @@ class BaseCriterion(object):
   def form_filter_args(self):
     raise NotImplementedError
 
+  @property
+  def form_unset_value(self):
+    raise NotImplementedError
+
 
 class TextSearchCriterion(BaseCriterion):
   """
search criterion: add 'form_unset_value'

this can help app & UI to detect if there is a filter currently applied