diff --git a/spacy/syntax/iterators.pyx b/spacy/syntax/iterators.pyx
index 75610ca8e..29cdbf89e 100644
--- a/spacy/syntax/iterators.pyx
+++ b/spacy/syntax/iterators.pyx
@@ -48,9 +48,9 @@ def english_noun_chunks(obj):
 def german_noun_chunks(obj):
     labels = ['sb', 'oa', 'da', 'nk', 'mo', 'ag', 'ROOT', 'root', 'cj', 'pd', 'og', 'app']
     doc = obj.doc # Ensure works on both Doc and Span.
-    np_label = doc.vocab.strings['NP']
-    np_deps = set(doc.vocab.strings[label] for label in labels)
-    close_app = doc.vocab.strings['nk']
+    np_label = doc.vocab.strings.add('NP')
+    np_deps = set(doc.vocab.strings.add(label) for label in labels)
+    close_app = doc.vocab.strings.add('nk')
 
     rbracket = 0
     for i, word in enumerate(obj):