Using Casadi from Julia

I am using Casadi from Julia and have the following code:

```
using PyCall
cd = pyimport("casadi")
opti = cd.Opti()
x = opti.variable()
```

This results in `ValueError('call stack is not deep enough')`.

However, the following workaround works:

```
using PyCall
cd = pyimport("casadi")
opti = cd.Opti()
x = py"$(opti).variable()"
```

Also the code works fine when run directly in Python. What is the reason for this?