dosubot[bot] commented on issue #3446:
URL: https://github.com/apache/hugegraph/issues/3446#issuecomment-6554598871

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   <!-- Answer -->
   A workaround to try is to give missing values an explicit, non-null bucket:
   
   ```groovy
   g.V('v1').groupCount().by(
       __.coalesce(__.values('d1'), __.constant('__missing__'))
   )
   ```
   
   TinkerPop 3.5.1 documents that `coalesce()` selects the first child 
traversal that emits a value, and `groupCount().by(...)` uses that projection 
as the grouping key. For your example, this should produce `{"__missing__":1}` 
rather than a null-keyed map. Choose a sentinel that cannot collide with real 
property values. I haven't executed this against 1.7.0. 
[[1]](https://tinkerpop.apache.org/docs/3.5.1/reference/#groupcount-step)
   
   The exception text also pinpoints the serialization boundary: HugeGraph's 
`JsonUtil.toJson()` calls Jackson's `writeValueAsString()` and wraps 
serialization failures as `HugeException("Can't write json: ...")`. This 
supports your distinction between constructing the traversal result and failing 
to encode it, though it does not establish where the null grouping key 
originates. 
[[2]](https://github.com/apache/hugegraph/blob/8ca41e2fc14d9750f835533b5bc7592415d4aa11/hugegraph-server/hugegraph-core/src/main/java/org/apache/hugegraph/util/JsonUtil.java#L67-L73)
   
   
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