diff --git a/mellon/utils.py b/mellon/utils.py
index 6462f81..1c48f1a 100644
--- a/mellon/utils.py
+++ b/mellon/utils.py
@@ -255,11 +255,18 @@ def login(request, user):
 
 def get_xml_encoding(content):
     xml_encoding = 'utf-8'
+
     def xmlDeclHandler(version, encoding, standalone):
-        xml_encoding = encoding
+        global xml_encoding
+
+        if encoding:
+            xml_encoding = encoding
     parser = expat.ParserCreate()
     parser.XmlDeclHandler = xmlDeclHandler
-    parser.Parse(content, True)
+    try:
+        parser.Parse(content, True)
+    except expat.ExpatError as e:
+        raise ValueError('invalid XML %s' % e)
     return xml_encoding
 
 
diff --git a/tests/test_sso_slo.py b/tests/test_sso_slo.py
index 276c373..42ab8ef 100644
--- a/tests/test_sso_slo.py
+++ b/tests/test_sso_slo.py
@@ -115,7 +115,7 @@ class MockIdp(object):
             del self.artifact_message
         login.buildResponseMsg()
         assert 'rsa-sha256' in login.msgBody
-        return login.msgBody
+        return '<?xml version="1.0"?>\n' + login.msgBody
 
     def mock_artifact_resolver(self):
         @all_requests