I am trying to fit the following model:

```
import pymc3 as pm
basic_model = pm.Model()
with basic_model:
exposures = pm.HalfNormal('exposures',sigma=2000,shape=(nSigs,nSamps))
y = pm.math.dot(sigs,exposures)
Y_obs = pm.Poisson('Y_obs',mu=y,observed=k)
trace = pm.sample(1000,cores=12,return_inferencedata=False)
```

My observations, k, range from 0 to 1,000,000 with mean around 2000. sigs are values on (0,1) and sum to 1 across columns. When I run the model, it gets about 25% of the way through and then crashes with:

ValueError: Mass matrix contains zeros on the diagonal.

The derivative of RV `exposures_log__`

.ravel()[2] is zero.

The derivative of RV `exposures_log__`

.ravel()[15] is zero.

The derivative of RV `exposures_log__`

.ravel()[21] is zero.

The derivative of RV `exposures_log__`

.ravel()[38] is zero.

Any ideas?