# Suddenly KeyError 'message' after running without problems

**URL:** <https://discourse.mc-stan.org/t/suddenly-keyerror-message-after-running-without-problems/31915>\
**Category:** Modeling\
**Created:** [June 29, 2023, 1:10pm UTC](https://discourse.mc-stan.org/t/suddenly-keyerror-message-after-running-without-problems/31915 "2023-06-29T13:10:59Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![HJAM24](https://avatars.discourse-cdn.com/v4/letter/h/ce73a5/32.png) [@HJAM24](https://discourse.mc-stan.org/u/HJAM24)\
**Post date:** [June 29, 2023, 1:10pm UTC](https://discourse.mc-stan.org/t/suddenly-keyerror-message-after-running-without-problems/31915/1 "2023-06-29T13:10:59Z")

</div>

Hi,

Since today I am suddenly getting a `KeyError` when I run the below code

```plaintext
model = stan.build(open('model.stan').read(), data)
stan_fit = model.sample(num_chains=4, num_samples=500)

```

Everything worked fine before. The full error traceback

```plaintext

---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
Cell In[51], line 1
----> 1 model = stan.build(open(
      2 'model.stan').read(), data)
      3 stan_fit = model.sample(num_chains=4, num_samples=500)

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/stan/model.py:519, in build(program_code, data, random_seed)
    516 return Model(model_name, program_code, data, param_names, constrained_param_names, dims, random_seed)
    518 try:
--> 519 return asyncio.run(go())
    520 except KeyboardInterrupt:
    521 return

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/nest_asyncio.py:35, in _patch_asyncio.<locals>.run(main, debug)
     33 task = asyncio.ensure_future(main)
     34 try:
---> 35 return loop.run_until_complete(task)
     36 finally:
     37 if not task.done():

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/nest_asyncio.py:90, in _patch_loop.<locals>.run_until_complete(self, future)
     87 if not f.done():
     88 raise RuntimeError(
     89 'Event loop stopped before Future completed.')
---> 90 return f.result()

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/futures.py:203, in Future.result(self)
    201 self.__log_traceback = False
    202 if self._exception is not None:
--> 203 raise self._exception.with_traceback(self._exception_tb)
    204 return self._result

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/asyncio/tasks.py:267, in Task.__step( ***failed resolving arguments*** )
    263 try:
    264 if exc is None:
    265 # We use the `send` method directly, because coroutines
    266 # don't have ` __iter__ ` and ` __next__ ` methods.
--> 267 result = coro.send(None)
    268 else:
    269 result = coro.throw(exc)

File /Library/Frameworks/Python.framework/Versions/3.11/lib/python3.11/site-packages/stan/model.py:511, in build.<locals>.go()
    509 resp = await client.post(f"/{model_name}/params", json={"data": data})
    510 if resp.status != 200:
--> 511 raise RuntimeError(resp.json()["message"])
    512 params_list = resp.json()["params"]
    513 assert len({param["name"] for param in params_list}) == len(params_list)

KeyError: 'message'

```

I am using Python 3.11, Pystan 3.7, asyncio 3.4.3 in a Jupyter Notebook.

Any idea why this is happening suddenly? Thank you

---

<div class="post-metadata">

**Author:** ![HJAM24](https://avatars.discourse-cdn.com/v4/letter/h/ce73a5/32.png) [@HJAM24](https://discourse.mc-stan.org/u/HJAM24)\
**Post date:** [June 29, 2023, 1:51pm UTC](https://discourse.mc-stan.org/t/suddenly-keyerror-message-after-running-without-problems/31915/2 "2023-06-29T13:51:24Z")

</div>

Apparently it is caused by a known [bug](https://github.com/stan-dev/pystan/issues/379).

---

<div class="post-metadata">

**Author:** ![Bob\_Carpenter](https://yyz2.discourse-cdn.com/flex030/user_avatar/discourse.mc-stan.org/bob_carpenter/32/9230_2.png) [@Bob\_Carpenter](https://discourse.mc-stan.org/u/Bob_Carpenter)\
**Post date:** [July 25, 2023, 8:08pm UTC](https://discourse.mc-stan.org/t/suddenly-keyerror-message-after-running-without-problems/31915/3 "2023-07-25T20:08:43Z")

</div>

I would suggest using cmdstanpy:

[https://cmdstanpy.readthedocs.io/en/v1.1.0/](https://cmdstanpy.readthedocs.io/en/v1.1.0/)