diff --git a/spacy/en/pos.pyx b/spacy/en/pos.pyx
index d8d1685b2..f1ce82390 100644
--- a/spacy/en/pos.pyx
+++ b/spacy/en/pos.pyx
@@ -252,7 +252,7 @@ cdef class EnPosTagger:
                 scores = self.model.score(context)
                 tokens.data[i].tag = arg_max(scores, self.model.n_classes)
                 self.set_morph(i, tokens.data)
-        tokens._tag_strings = self.tag_names
+        tokens._tag_strings = tuple(self.tag_names)
         tokens.is_tagged = True
 
     def train(self, Tokens tokens, object gold_tag_strs):