Index: Lib/test/test_minidom.py
===================================================================
--- Lib/test/test_minidom.py	(revision 60745)
+++ Lib/test/test_minidom.py	(working copy)
@@ -1306,6 +1306,11 @@
                         and n2.ownerDocument.isSameNode(doc2))
             for i in range(len(n1.childNodes)):
                 stack.append((n1.childNodes[i], n2.childNodes[i]))
+    
+    def testSerializeCommentNodeWithDoubleHyphen(self):
+        doc = create_doc_without_doctype()
+        doc.appendChild(doc.createComment("foo--bar"))
+        self.assertRaises(ValueError, doc.toxml)
 
 def test_main():
     run_unittest(MinidomTest)
Index: Lib/xml/dom/minidom.py
===================================================================
--- Lib/xml/dom/minidom.py	(revision 60745)
+++ Lib/xml/dom/minidom.py	(working copy)
@@ -1126,6 +1126,8 @@
         self.data = self.nodeValue = data
 
     def writexml(self, writer, indent="", addindent="", newl=""):
+        if "--" in self.data:
+            raise ValueError("'--' is not allowed in a comment node")
         writer.write("%s<!--%s-->%s" % (indent, self.data, newl))