diff --git a/sat/plugins/plugin_xep_0059.py b/sat/plugins/plugin_xep_0059.py
index 080f5f0d..721f140c 100644
--- a/sat/plugins/plugin_xep_0059.py
+++ b/sat/plugins/plugin_xep_0059.py
@@ -61,6 +61,9 @@ class XEP_0059(object):
         @return (rsm.RSMRequest, None): request with parsed arguments
             or None if no RSM arguments have been found
         """
+        if int(extra.get(RSM_PREFIX + u'max', 0)) < 0:
+            raise ValueError(_(u"rsm_max can't be negative"))
+
         rsm_args = {}
         for arg in (u"max", u"after", u"before", u"index"):
             try:
diff --git a/sat/plugins/plugin_xep_0313.py b/sat/plugins/plugin_xep_0313.py
index e0307ddf..6063c027 100644
--- a/sat/plugins/plugin_xep_0313.py
+++ b/sat/plugins/plugin_xep_0313.py
@@ -143,7 +143,7 @@ class XEP_0313(object):
 
         for name, value in extra.iteritems():
             if name.startswith(FILTER_PREFIX):
-                var = name[len(FILTER_PREFIX) :]
+                var = name[len(FILTER_PREFIX):]
                 extra_fields = form_args.setdefault(u"extra_fields", [])
                 extra_fields.append(data_form.Field(var=var, value=value))