# Need help with debugging: Getting batch size of 0

**URL:** <https://discuss.ray.io/t/need-help-with-debugging-getting-batch-size-of-0/3762>\
**Category:** RLlib\
**Created:** [October 8, 2021, 9:00pm UTC](https://discuss.ray.io/t/need-help-with-debugging-getting-batch-size-of-0/3762 "2021-10-08T21:00:37Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Aceticia](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ray.io/aceticia/32/1506_2.png) [@Aceticia](https://discuss.ray.io/u/Aceticia)\
**Post date:** [October 8, 2021, 9:00pm UTC](https://discuss.ray.io/t/need-help-with-debugging-getting-batch-size-of-0/3762/1 "2021-10-08T21:00:38Z")

</div>

I’ve been having some issues with getting a custom env and model to run. The observation spaces and action spaces are both tuples of simpler spaces. The bug is that during learning, some batches have a length of 0. These batches have `seq_len` that are normal and containing correct sizes, but the data (obs, act, prev\_act…) all have batch sizes of 0. This exact effect can be traced back to this line in `_slice` method of `SampleBatch`:

```auto
data = tree.map_structure_with_path(map_, self)

```

The function `map_` uses the lower bound and upper bound `start:stop` for data and `start_seq_len:stop_seq_len` for `seq_len`. By manually running this a few times, I found that despite start and stop never being identical, this line sometimes still produces batch sizes of 0. Here is the code I used to print diagnostic information right after this line:

```auto
print(start, stop)
print(len(data))
print(data[SampleBatch.OBS][0].shape)

```

And this is one output showing the discrepancy in size:

```auto
(PPO pid=29078) 4700 4820
(PPO pid=29078) 120
(PPO pid=29078) (120, 210, 160, 3)
(PPO pid=29078) 5520 5660
(PPO pid=29078) 121
(PPO pid=29078) (0, 210, 160, 3) # Batch size becomes 0

```

Could this be a bug from my own code? Or is this a bug from inside RLLib about handling complex space? I’m not sure if I should keep looking into the `tree` package or in my own code. Any comments/tips would be helpful here, thanks a lot!!

---

<div class="post-metadata">

**Author:** ![Aceticia](https://sea2.discourse-cdn.com/flex020/user_avatar/discuss.ray.io/aceticia/32/1506_2.png) [@Aceticia](https://discuss.ray.io/u/Aceticia)\
**Post date:** [October 8, 2021, 9:22pm UTC](https://discuss.ray.io/t/need-help-with-debugging-getting-batch-size-of-0/3762/2 "2021-10-08T21:22:18Z")

</div>

Marking this as resolved as I found that the `_slice` method got indices that are outside of its length.