# Laplace approximation TypeError

**URL:** <https://discourse.pymc.io/t/laplace-approximation-typeerror/17342>\
**Category:** General\
**Created:** [September 22, 2025, 1:19pm UTC](https://discourse.pymc.io/t/laplace-approximation-typeerror/17342 "2025-09-22T13:19:28Z")\
**Posts on this page:** 3\
**Page:** 1

<div class="post-metadata">

**Author:** ![urikokp](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/urikokp/32/9088_2.png) [@urikokp](https://discourse.pymc.io/u/urikokp)\
**Post date:** [September 22, 2025, 1:19pm UTC](https://discourse.pymc.io/t/laplace-approximation-typeerror/17342/1 "2025-09-22T13:19:28Z")

</div>

Hello. I would like to use the Laplace Approximation in **pymc-extras**. Both MCMC and ADVI work fine, but LA throws a **TypeError** as shown below. The source code is too complicated for me to figure out the cause. Below are my model and the error message. What is causing this error?

### Model

```auto
from patsy import dmatrix

# blossom_data = pd.read_csv(pm.get_data("cherry_blossoms.csv"), sep=";")
blossom_data = pd.read_csv('./data/cherry_blossoms.csv')
blossom_data = blossom_data.dropna(subset=["doy"]).reset_index(drop=True)

num_knots = 15
knot_list = np.percentile(blossom_data.year, np.linspace(0, 100, num_knots + 2))[1:-1]
B_cov = dmatrix(
    "bs(year, knots=knots, degree=3, include_intercept=True) - 1",
    {"year": blossom_data.year.values, "knots": knot_list},
)
B_cov = np.asarray(B_cov)
Y = blossom_data.doy.values

COORDS = {"splines": np.arange(B_cov.shape[1])}
with pm.Model(coords=COORDS) as pymc_model:
    obs = pm.Data('obs', Y)
    B = pm.Data('B', B_cov)

    a = pm.Normal("a", 100, 5, shape=(1,))
    w = pm.Normal("w", mu=0, sigma=3, dims="splines")

    mu = pm.Deterministic("mu", a + pt.dot(B, w))
    sigma = pm.Exponential("sigma", 1, shape=(1,))

    _ = pm.Normal("y", mu=mu, sigma=sigma, observed=obs)

```

### Run Code

```auto
with pymc_model:
    idata = pmx.fit(
        method="laplace",
        chains=4,
        draws=2500,
    )

```

### Error Message

```auto
File ~/.conda/envs/pymc_env3/lib/python3.13/site-packages/pymc_extras/inference/laplace_approx/laplace.py:223, in model_to_laplace_approx(model, unpacked_variable_names, chains, draws)
    220 initval = initial_point.get(name, None)
    221 dim_shapes = initval.shape if initval is not None else batched_rv.type.shape[2:]
    222 laplace_model.add_coords(
--> 223 {name: np.arange(shape) for name, shape in zip(dims[2:], dim_shapes)}
    224 )
    226 pm.Deterministic(name, batched_rv, dims=dims)
    228 return laplace_model

TypeError: unsupported operand type(s) for -: 'NoneType' and 'int'

```

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<div class="post-metadata">

**Author:** ![jessegrabowski](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/jessegrabowski/32/5010_2.png) [@jessegrabowski](https://discourse.pymc.io/u/jessegrabowski)\
**Post date:** [September 23, 2025, 12:37am UTC](https://discourse.pymc.io/t/laplace-approximation-typeerror/17342/2 "2025-09-23T00:37:05Z")

</div>

This is a bug. I opened an issue about it [here](https://github.com/pymc-devs/pymc-extras/issues/581). You can side-step it by setting coords on `mu`:

```py
COORDS = {"splines": np.arange(B_cov.shape[1]), 
          'obs_idx': np.arange(Y.shape[0])}

with pm.Model(coords=COORDS) as pymc_model:
    obs = pm.Data('obs', Y, dims=['obs_idx'])
    B = pm.Data('B', B_cov)

    a = pm.Normal("a", 100, 5, shape=(1,))
    w = pm.Normal("w", mu=0, sigma=3, dims="splines")

    mu = pm.Deterministic("mu", a + pt.dot(B, w), dims=['obs_idx'])
    sigma = pm.Exponential("sigma", 1, shape=(1,))

    _ = pm.Normal("y", mu=mu, sigma=sigma, observed=obs)

```

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<div class="post-metadata">

**Author:** ![urikokp](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/urikokp/32/9088_2.png) [@urikokp](https://discourse.pymc.io/u/urikokp)\
**Post date:** [September 23, 2025, 4:50am UTC](https://discourse.pymc.io/t/laplace-approximation-typeerror/17342/3 "2025-09-23T04:50:16Z")

</div>

Thank you so much!