SET unmapped_fields="nullify";
from employees
| FORK  (where foo != 84) (where true) 
| WHERE _fork == "fork1" 
| DROP _fork 
| eval y = coalesce(bar, baz) 

                "type": "illegal_state_exception",
                "reason": "Found 1 problem\nline 9:3: Plan [Filter[_fork{r}#8044 == fork1[KEYWORD]]] optimized incorrectly due to missing references [_fork{r}#8044]",
                "stack_trace": "org.elasticsearch.ElasticsearchException$1: Found 1 problem\nline 9:3: Plan [Filter[_fork{r}#8044 == fork1[KEYWORD]]] optimized incorrectly due to missing references [_fork{r}#8044]