diff --git a/spacy/syntax/ner.pyx b/spacy/syntax/ner.pyx
index c2597bc7e..0a40ed9a8 100644
--- a/spacy/syntax/ner.pyx
+++ b/spacy/syntax/ner.pyx
@@ -76,6 +76,8 @@ cdef class BiluoPushDown(TransitionSystem):
         for (raw_text, toks, (ids, words, tags, heads, labels, biluo)) in gold_tuples:
             for i, ner_tag in enumerate(biluo):
                 if ner_tag != 'O' and ner_tag != '-':
+                    if ner_tag.count('-') != 1:
+                        raise ValueError(ner_tag)
                     _, label = ner_tag.split('-')
                     for move_str in ('B', 'I', 'L', 'U'):
                         move_labels[moves.index(move_str)][label] = True