QUESTION: Node Constraints on Property

Is there a way to create constraints for node/vertex properties? I want to use concurrent loading of AGE but need constraints to avoid multiple threads from creating duplicate nodes/vertex. I tried this:

```CREATE UNIQUE INDEX mesh_name_ids
ON pubmed."Article"
((properties->>'pmid'));
```

but I get this error:
```
operator does not exist: ag_catalog.agtype ->> unknown
Hint: No operator matches the given name and argument types. You might need to add explicit type casts.
```

Are constraints on a Node property possible?