diff --git a/CHANGES.txt b/CHANGES.txt
index 0fe1700..af03b6f 100644
--- a/CHANGES.txt
+++ b/CHANGES.txt
@@ -21,6 +21,10 @@ Changelog
   default type is set properly when creating a new comment.
   [timo]
 
+- Fix handling of comments with invalid transforms. Write an error msg
+  to the log and just return the untransformed text.
+  [timo]
+
 
 2.1.8 (unreleased)
 ------------------
diff --git a/plone/app/discussion/comment.py b/plone/app/discussion/comment.py
index 7b4c12c..171f4df 100644
--- a/plone/app/discussion/comment.py
+++ b/plone/app/discussion/comment.py
@@ -142,10 +142,22 @@ class Comment(CatalogAware, WorkflowAware, DynamicType, Traversable,
             return ''
         if isinstance(text, unicode):
             text = text.encode('utf8')
-        return transforms.convertTo(targetMimetype,
-                                    text,
-                                    context=self,
-                                    mimetype=sourceMimetype).getData()
+        transform = transforms.convertTo(
+            targetMimetype,
+            text,
+            context=self,
+            mimetype=sourceMimetype)
+        if transform:
+            return transform.getData()
+        else:
+            logger = logging.getLogger("plone.app.discussion")
+            logger.error(
+                _(u"Transform '%s' => '%s' not available. Failed to transform comment '%s'." % (
+                    sourceMimetype,
+                    targetMimetype,
+                    self.absolute_url(),
+                    )))
+            return text
 
     def Title(self):
         """The title of the comment.
diff --git a/plone/app/discussion/tests/test_comment.py b/plone/app/discussion/tests/test_comment.py
index 3afe87e..5a4deb1 100644
--- a/plone/app/discussion/tests/test_comment.py
+++ b/plone/app/discussion/tests/test_comment.py
@@ -177,6 +177,16 @@ class CommentTest(unittest.TestCase):
         comment1.text = 'para'
         self.assertEqual(comment1.getText(targetMimetype='text/plain'), 'para')
 
+    def test_getText_invalid_transformation_raises_error(self):
+        conversation = IConversation(self.portal.doc1)
+        comment1 = createObject('plone.Comment')
+        comment1.mime_type = 'text/x-html-safe'
+        comment1.text = 'para'
+        conversation.addComment(comment1)
+        self.assertEqual(
+            comment1.getText(targetMimetype='text/html'),
+            'para')
+
     def test_traversal(self):
         # make sure comments are traversable, have an id, absolute_url and
         # physical path