don't do anything if add_anchor fails

IPython/nbconvert/filters/strings.py

@@ -72,7 +72,11 b' def html2text(element):'
     Analog of jQuery's $(element).text()
     """
     if isinstance(element, py3compat.string_types):
-        element = ElementTree.fromstring(element)
+        try:
+            element = ElementTree.fromstring(element)
+        except Exception:
+            # failed to parse, just return it unmodified
+            return element
     
     text = element.text or ""
     for child in element:
@@ -86,7 +90,11 b' def add_anchor(html):'
     
     For use in heading cells
     """
-    h = ElementTree.fromstring(py3compat.cast_bytes_py2(html, encoding='utf-8'))
+    try:
+        h = ElementTree.fromstring(py3compat.cast_bytes_py2(html, encoding='utf-8'))
+    except Exception:
+        # failed to parse, just return it unmodified
+        return html
     link = html2text(h).replace(' ', '-')
     h.set('id', link)
     a = ElementTree.Element("a", {"class" : "anchor-link", "href" : "#" + link})

IPython/nbconvert/filters/tests/test_strings.py

@@ -60,7 +60,12 b' class TestStrings(TestsBase):'
         assert '<b' in results
         assert '</b>' in results
 
-        
+    def test_add_anchor_fail(self):
+        """add_anchor does nothing when it fails"""
+        html = '<h1>Hello <br>World!</h1>'
+        results = add_anchor(html)
+        self.assertEqual(html, results)
+
     def test_strip_dollars(self):
         """strip_dollars test"""
         tests = [