diff --git a/src/tools/voc_eval_lib/datasets/voc_eval.py b/src/tools/voc_eval_lib/datasets/voc_eval.py
index 2fda402..5a49b43 100644
--- a/src/tools/voc_eval_lib/datasets/voc_eval.py
+++ b/src/tools/voc_eval_lib/datasets/voc_eval.py
@@ -119,7 +119,7 @@ def voc_eval(detpath,
           i + 1, len(imagenames)))
     # save
     print('Saving cached annotations to {:s}'.format(cachefile))
-    with open(cachefile, 'w') as f:
+    with open(cachefile, 'wb') as f:
       pickle.dump(recs, f)
   else:
     # load