diff --git a/common/lib/capa/capa/responsetypes.py b/common/lib/capa/capa/responsetypes.py
index 5aa5b917d8..97077ace8f 100644
--- a/common/lib/capa/capa/responsetypes.py
+++ b/common/lib/capa/capa/responsetypes.py
@@ -174,7 +174,7 @@ class LoncapaResponse(object):
         '''
         return sum(self.maxpoints.values())
 
-    def render_html(self, renderer, response_msg=None):
+    def render_html(self, renderer, response_msg=''):
         '''
         Return XHTML Element tree representation of this Response.