Hi,

Consider a dummy example:

```
@config_enumerate()
def model(a=None, b=None):
a = sample("a", dist.Bernoulli(0.75), obs=a)
p = 1-0.25*a
b = sample("b", dist.Bernoulli(p), obs=b)
```

Can anyone help me to understand why two lines below are working as expected:

```
elbo = pyro.infer.TraceEnum_ELBO(max_plate_nesting=1)
margin = elbo.compute_marginals(model, guide, b=tt(1.))
```

while:

```
margin = elbo.compute_marginals(model, guide, a=tt(1.))
margin = elbo.compute_marginals(model, guide)
```

fails with error:

```
ValueError: Number of einsum subscripts must be equal to the number of operands.
```

How to get elbo.compute_marginals(model, guide, a=tt(1.)) running?

Cheers!