diff --git a/common/lib/capa/capa/tests/response_xml_factory.py b/common/lib/capa/capa/tests/response_xml_factory.py
index de4c17bae3..af62ef715c 100644
--- a/common/lib/capa/capa/tests/response_xml_factory.py
+++ b/common/lib/capa/capa/tests/response_xml_factory.py
@@ -102,7 +102,7 @@ class ResponseXMLFactory(six.with_metaclass(ABCMeta, object)):
                 explanation_div.set("class", "detailed-solution")
                 explanation_div.text = explanation_text
 
-        return etree.tostring(root)
+        return etree.tostring(root).decode('utf-8')
 
     @staticmethod
     def textline_input_xml(**kwargs):