I am trying to use an sqlite backend but it raises

```
ProgrammingError: Cannot operate on a closed database.
```

Here is a minimal working example:

```
import pymc3 as pm
from pymc3.backends import SQLite
import numpy as np
#simulate some data
x = np.random.normal(100)
y = x + np.random.normal(100)
with pm.Model() as model:
sigma = pm.HalfCauchy('sigma', beta=10, testval=1.)
intercept = pm.Normal('Intercept', 0, sigma=20)
x_coeff = pm.Normal('x', 0, sigma=20)
# Define likelihood
likelihood = pm.Normal('y', mu=intercept + x_coeff * x,
sigma=sigma, observed=y)
backend = SQLite('mod_trace.sqlite')
trace2 = pm.sample(draws=5000, tune=3000, target_accept=.95, trace=backend)
```

Am I doing this wrong? This was meant to be a workaround for a model in which pymc3 is using up all my memory and freezing my computer when I run it for very long.