diff --git a/exp/kerokero/prepare_data.py b/exp/kerokero/prepare_data.py
--- a/exp/kerokero/prepare_data.py
+++ b/exp/kerokero/prepare_data.py
@@ -2,7 +2,6 @@ import os
 import sys
 import re
 import random
-import itertools
 
 import numpy as np
 import cv2 as cv
@@ -40,7 +39,7 @@ class Sample:
 		img=cv.warpPerspective(self.img,m,(self.SIDE,self.SIDE))
 		img=np.float32(img)/128-1
 		grid=Corners(c.transform(m) for c in self.grid)
-		grid=list(map(lambda p: 2*p/self.SIDE-EPoint(1,1), grid))
+		grid=list(map(lambda p: list(2*p/self.SIDE-EPoint(1,1)), grid))
 		return (img,grid)
 
 	def _getCenter(self):