Throw ClassCastException

I first randomly create a graph. Then when I run the following query: `g.V().in('el1').as('x').V().both('el0','el1').inE('el1').outV().where(within('x'))` is thrown with an exception. I think this query is syntactically correct, but I keep triggering this kind of problem. I generate the query based on the rule which can be refered from: https://stackoverflow.com/questions/48067834/gremlin-intersection-operation. Following this instruction, I think it's allowed to generate this kind of queries.

Expected behavior:
No exception should be expected to throw. Or futher messages or prompts should be thrown.

Actual behavior:
A `java.util.concurrent.ExecutionException` is thrown. And I'm not really sure whether this problem should happen so I report this.
```
java.util.concurrent.ExecutionException: org.apache.tinkerpop.gremlin.driver.exception.ResponseException: org.apache.tinkerpop.gremlin.tinkergraph.structure.TinkerVertex cannot be cast to java.util.Collection
```