Sorry, this was a miss on my part, we should not be setting any key names to `k.expression`, we should be using `set_value_from_jmespath`.  For example if you run `aws rds describe-engine-default-parameters --db-parameter-group-family mysql5.1`  you'll see that the result is:

```
{
    "EngineDefaults": {
        "Parameters": []
    },
    "EngineDefaults.Parameters": [
        {
            "Description": "Controls whether user-defined functions that have only an xxx symbol for the main function can be loaded",
        ...
        }
}
```