diff --git a/parserator/data_prep_utils.py b/parserator/data_prep_utils.py
index 6f36dd1..0d5b635 100644
--- a/parserator/data_prep_utils.py
+++ b/parserator/data_prep_utils.py
@@ -51,6 +51,13 @@ def _strip_formatting(self, xml):
 
     def __iter__(self):
         for sequence_xml in self.xml:
+            # Skip XML comments (handy for grouping examples in the same
+            # file) and sequences that ended up with no children, both of
+            # which would otherwise blow up trainModel downstream.
+            if isinstance(sequence_xml, etree._Comment):
+                continue
+            if len(sequence_xml) == 0:
+                continue
             raw_text = etree.tostring(sequence_xml, method="text", encoding="unicode")
             yield raw_text, self._xml_to_sequence(sequence_xml)
 
diff --git a/tests/test_xml.py b/tests/test_xml.py
index c8b23bd..0e4dff2 100644
--- a/tests/test_xml.py
+++ b/tests/test_xml.py
@@ -51,5 +51,33 @@ def XMLequals(self, labeled_sequence, xml):
         assert correct_xml == generated_xml
 
 
+class TestTrainingDataIter(unittest.TestCase):
+    def _td(self, xml_str):
+        return data_prep_utils.TrainingData(xml=etree.fromstring(xml_str))
+
+    def test_skips_comments(self):
+        # Comments are useful for grouping examples in the same file; they
+        # used to leak into __iter__ and crash trainModel.
+        td = self._td(
+            "<Collection>"
+            "<!-- group one -->"
+            "<TokenSequence><foo>a</foo></TokenSequence>"
+            "<!-- group two -->"
+            "<TokenSequence><foo>b</foo></TokenSequence>"
+            "</Collection>"
+        )
+        assert [raw for raw, _ in td] == ["a", "b"]
+
+    def test_skips_empty_sequences(self):
+        td = self._td(
+            "<Collection>"
+            "<TokenSequence><foo>a</foo></TokenSequence>"
+            "<TokenSequence></TokenSequence>"
+            "<TokenSequence><foo>b</foo></TokenSequence>"
+            "</Collection>"
+        )
+        assert [raw for raw, _ in td] == ["a", "b"]
+
+
 if __name__ == "__main__":
     unittest.main()