diff --git a/spacy/serialize.pyx b/spacy/serialize.pyx
index 148ac9113..a2ffec62e 100644
--- a/spacy/serialize.pyx
+++ b/spacy/serialize.pyx
@@ -31,6 +31,8 @@ cdef struct Node:
 cdef class HuffmanCodec:
     cdef vector[Node] nodes
     cdef vector[vector[bint]] codes
+    cdef vector[bint] oov_code
+    cdef uint64_t oov_symbol
     cdef float[:] probs
     cdef dict table
     def __init__(self, symbols, probs):
@@ -44,11 +46,15 @@ cdef class HuffmanCodec:
         cdef vector[bint] path
         assign_codes(self.nodes, self.codes, len(self.nodes) - 1, path)
 
-    def encode(self, sequence):
+    def encode(self, uint64_t[:] sequence):
         bits = []
+        cdef uint64_t symbol
         for symbol in sequence:
-            i = self.table[symbol]
-            code = self.codes[i]
+            i = <int>self.table.get(symbol)
+            if i == 0:
+                raise Exception("Unseen symbol: %s" % symbol)
+            else:
+                code = self.codes[i]
             bits.extend(code)
         return bits