the value should be a list to unpacking (#688)

utils/datasets.py

@@ -457,7 +457,7 @@ class LoadImagesAndLabels(Dataset):  # for training/testing
                     l = np.zeros((0, 5), dtype=np.float32)
                 x[img] = [l, shape]
             except Exception as e:
-                x[img] = None
+                x[img] = [None, None]
                 print('WARNING: %s: %s' % (img, e))
 
         x['hash'] = get_hash(self.label_files + self.img_files)