diff --git a/fairseq/data/dictionary.py b/fairseq/data/dictionary.py
index 745db16e43..9b893c858e 100644
--- a/fairseq/data/dictionary.py
+++ b/fairseq/data/dictionary.py
@@ -131,7 +131,7 @@ def add_symbol(self, word, n=1, overwrite=True):
             self.count[idx] = self.count[idx] + n
             return idx
         else:
-            idx = len(self.indices)
+            idx = len(self.symbols)
             self.indices[word] = idx
             self.symbols.append(word)
             self.count.append(n)