Hello,

I’m trying to learn Stan by slowly implementing a poisson GLM. I’m starting with just getting the mean to sample and I’m getting the following error:

```
RuntimeError: Exception: mismatch in number dimensions declared and found in context; processing stage=data initialization; variable name=y; dims declared=(1233565); dims found=(1233565,1) (in 'unknown file name' at line 4)
```

Below is my model:

When including Stan code in your post it really helps if you make it as readable as possible by using Stan code chunks (```stan) with clear spacing and indentation. For example, use

```
liability_code = """
data {
int<lower=0> N;
vector[N] y;
}
parameters { //parameters is what we want to infer....we don't know the coefficients
real mu;
}
model {
y ~ poisson(mu);
}
```

```
stan_model = pystan.stan(model_code=liability_code, data = {'N':1233565, 'y':y}, iter = 1000, chains = 2)
```

Eventually, I have a data frame of 18 variables that go into this model. I’m trying to show what a Bayesian analysis can do for our team to my boss. Thanks for any help getting me there.