diff --git a/spacy/utf8string.pxd b/spacy/utf8string.pxd
index 6bd5c6757..5ef4113d5 100644
--- a/spacy/utf8string.pxd
+++ b/spacy/utf8string.pxd
@@ -31,4 +31,4 @@ cdef class StringStore:
     cdef int size
     cdef int _resize_at
     
-    cdef Utf8Str* intern(self, char* chars, int length) except NULL
+    cdef const Utf8Str* intern(self, char* chars, int length) except NULL
diff --git a/spacy/utf8string.pyx b/spacy/utf8string.pyx
index 0384a150c..1d2b7a264 100644
--- a/spacy/utf8string.pyx
+++ b/spacy/utf8string.pyx
@@ -20,7 +20,7 @@ cdef class StringStore:
 
     def __getitem__(self, object string_or_id):
         cdef bytes byte_string
-        cdef Utf8Str* utf8str
+        cdef const Utf8Str* utf8str
         if isinstance(string_or_id, int) or isinstance(string_or_id, long):
             if string_or_id < 1 or string_or_id >= self.size:
                 raise IndexError(string_or_id)
@@ -36,7 +36,7 @@ cdef class StringStore:
         else:
             raise TypeError(type(string_or_id))
 
-    cdef Utf8Str* intern(self, char* chars, int length) except NULL:
+    cdef const Utf8Str* intern(self, char* chars, int length) except NULL:
         # 0 means missing, but we don't bother offsetting the index. We waste
         # slot 0 to simplify the code, because it doesn't matter.
         assert length != 0