diff --git a/spacy/__init__.pxd b/spacy/__init__.pxd
new file mode 100644
index 000000000..e69de29bb
diff --git a/spacy/lang.pyx b/spacy/lang.pyx
index 0f8d21a24..d106f172a 100644
--- a/spacy/lang.pyx
+++ b/spacy/lang.pyx
@@ -155,8 +155,10 @@ cdef class Language:
         cdef Lexeme** lexemes
         cdef Lexeme* lexeme
         cdef String span
-        idx = tokens.extend(idx, prefixes.data(), prefixes.size())
+        if prefixes.size():
+            idx = tokens.extend(idx, prefixes.data(), prefixes.size())
         if string.n != 0:
+
             lexemes = <Lexeme**>self.cache.get(string.key)
             if lexemes != NULL:
                 idx = tokens.extend(idx, lexemes, 0)
diff --git a/spacy/pos_util.py b/spacy/pos_util.py
index 5b020eaed..f8aa8edb7 100644
--- a/spacy/pos_util.py
+++ b/spacy/pos_util.py
@@ -42,7 +42,8 @@ def _parse_line(line, sep):
         assert len(subtags) == len(subtokens), [t.string for t in subtokens]
         words.append(word)
         tags.extend([Tagger.encode_pos(ptb_to_univ(pos)) for pos in subtags])
-    return EN.tokenize(' '.join(words)), tags
+    tokens = EN.tokenize(' '.join(words)), tags
+    return tokens
 
 
 def get_tagdict(train_sents):
diff --git a/spacy/utf8string.pxd b/spacy/utf8string.pxd
new file mode 100644
index 000000000..82ae50022
--- /dev/null
+++ b/spacy/utf8string.pxd
@@ -0,0 +1,21 @@
+from preshed.maps cimport PreshMap
+from cymem.cymem cimport Pool
+
+from .typedefs cimport utf8_t, id_t, hash_t
+
+
+cdef struct Utf8Str:
+    id_t i
+    hash_t key
+    utf8_t chars
+    int length
+
+
+cdef class StringStore:
+    cdef Pool mem
+    cdef PreshMap table
+    cdef Utf8Str* strings
+    cdef int size
+    cdef int _resize_at
+    
+    cdef Utf8Str* intern(self, char* chars, int length) except NULL
diff --git a/spacy/utf8string.pyx b/spacy/utf8string.pyx
new file mode 100644
index 000000000..d3bc3a4fe
--- /dev/null
+++ b/spacy/utf8string.pyx
@@ -0,0 +1,53 @@
+from libc.string cimport memcpy
+
+from murmurhash.mrmr cimport hash64
+
+
+cdef class StringStore:
+    def __init__(self):
+        self.mem = Pool()
+        self.table = PreshMap()
+        self._resize_at = 10000
+        self.strings = <Utf8Str*>self.mem.alloc(self._resize_at, sizeof(Utf8Str))
+        self.size = 1
+
+    property size:
+        def __get__(self):
+            return self.size-1
+
+    def __getitem__(self, string_or_id):
+        cdef bytes byte_string
+        cdef Utf8Str* utf8str
+        if type(string_or_id) == int or type(string_or_id) == long:
+            if string_or_id < 1 or string_or_id >= self.size:
+                raise IndexError(string_or_id)
+            utf8str = &self.strings[<int>string_or_id]
+            return utf8str.chars[:utf8str.length]
+        elif type(string_or_id) == bytes:
+            utf8str = self.intern(<char*>string_or_id, len(string_or_id))
+            return utf8str.i
+        else:
+            raise TypeError(type(string_or_id))
+
+    cdef 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
+        cdef hash_t key = hash64(chars, length * sizeof(char), 0)
+        cdef void* value = self.table.get(key)
+        cdef size_t i
+        if value == NULL:
+            if self.size == self._resize_at:
+                self._resize_at *= 2
+                self.strings = <Utf8Str*>self.mem.realloc(self.strings, self._resize_at * sizeof(Utf8Str))
+            i = self.size
+            self.strings[i].i = self.size
+            self.strings[i].key = key
+            self.strings[i].chars = <char*>self.mem.alloc(length, sizeof(char))
+            memcpy(self.strings[i].chars, chars, length)
+            self.strings[i].length = length
+            self.table.set(key, <void*>self.size)
+            self.size += 1
+        else:
+            i = <size_t>value
+        return &self.strings[i]