diff --git a/src/tmp/wokkel/rsm.py b/src/tmp/wokkel/rsm.py
index cc1eef1b..5ece518e 100644
--- a/src/tmp/wokkel/rsm.py
+++ b/src/tmp/wokkel/rsm.py
@@ -117,6 +117,8 @@ class RSMRequest(object):
             after = None
         else:
             after = unicode(after_elt)
+            if not after:
+                raise RSMError("<after/> element can't be empty in RSM request")
 
         try:
             max_elt = set_elt.elements(NS_RSM, 'max').next()