diff --git a/spacy/morphology.pyx b/spacy/morphology.pyx
index e8b1f3520..442aebd68 100644
--- a/spacy/morphology.pyx
+++ b/spacy/morphology.pyx
@@ -38,6 +38,8 @@ cdef class Morphology:
             tag_id = self.reverse_index[self.strings[tag]]
         else:
             tag_id = tag
+        if tag_id >= self.n_tags:
+            raise ValueError("Unknown tag: %s" % tag)
         analysis = <MorphAnalysisC*>self._cache.get(tag_id, token.lex.orth)
         if analysis is NULL:
             analysis = <MorphAnalysisC*>self.mem.alloc(1, sizeof(MorphAnalysisC))
diff --git a/spacy/tokens/doc.pyx b/spacy/tokens/doc.pyx
index 9c3387059..6b14d761c 100644
--- a/spacy/tokens/doc.pyx
+++ b/spacy/tokens/doc.pyx
@@ -398,7 +398,8 @@ cdef class Doc:
                         self.is_parsed = True
             elif attr_id == TAG:
                 for i in range(length):
-                    self.vocab.morphology.assign_tag(&tokens[i], values[i])
+                    self.vocab.morphology.assign_tag(&tokens[i],
+                                                     self.vocab.strings[values[i]])
                     if not self.is_tagged and tokens[i].tag != 0:
                         self.is_tagged = True
             elif attr_id == POS: