diff --git a/knowledge/survey/questionnaire.py b/knowledge/survey/questionnaire.py
index 5b698e4..a12d519 100644
--- a/knowledge/survey/questionnaire.py
+++ b/knowledge/survey/questionnaire.py
@@ -98,7 +98,7 @@ class Response(object):
             score = scoreMax = 0.0
             for qu in qugroup.questions:
                 value = self.values.get(qu)
-                if value is None:
+                if value is None or isinstance(value, basestring):
                     continue
                 if qu.revertAnswerOptions:
                     value = qu.answerRange - value - 1