# GraphQL Error Propagation - Graphql

**URL:** <https://discuss.dgraph.io/t/graphql-error-propagation-graphql/9892>\
**Category:** Documentation\
**Created:** [August 29, 2020, 7:54am UTC](https://discuss.dgraph.io/t/graphql-error-propagation-graphql/9892 "2020-08-29T07:54:21Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![diggy](https://yyz1.discourse-cdn.com/flex007/user_avatar/discuss.dgraph.io/diggy/32/3666_2.png) [@diggy](https://discuss.dgraph.io/u/diggy)\
**Post date:** [August 29, 2020, 7:54am UTC](https://discuss.dgraph.io/t/graphql-error-propagation-graphql/9892/1 "2020-08-29T07:54:21Z")

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Before returning query and mutation results, Dgraph uses the types in the schema to apply GraphQL [value completion](https://graphql.github.io/graphql-spec/June2018/#sec-Value-Completion) and [error handling](https://graphql.github.io/graphql-spec/June2018/#sec-Errors-and-Non-Nullability). That is, `null` values for non-nullable fields, e.g. `String!`, cause error propagation to parent fields.

In short, the GraphQL value completion and error propagation mean the following.

- Fields marked as nullable (i.e. without `!`) can return `null` in the json response.
- For fields marked as non-nullable (i.e. with `!`) Dgraph never returns null for that field.
- If an instance of type has a non-nullable field that has evaluated to null, the whole instance results in null.
- Reducing an object to null might cause further error propagation. For example, querying for a post that has an author with a null name results in null: the null name (`name: String!`) causes the author to result in null, and a null author causes the post (`author: Author!`) to result in null.
- Error propagation for lists with nullable elements, e.g. `friends [Author]`, can result in nulls inside the result list.
- Error propagation for lists with non-nullable elements results in null for `friends [Author!]` and would cause further error propagation for `friends [Author!]!`.

Note that, a query that results in no values for a list will always return the empty list `[]`, not `null`, regardless of the nullability. For example, given a schema for an author with `posts: [Post!]!`, if an author has not posted anything and we queried for that author, the result for the posts field would be `posts: []`.

A list can, however, result in null due to GraphQL error propagation. For example, if the definition is `posts: [Post!]`, and we queried for an author who has a list of posts. If one of those posts happened to have a null title (title is non-nullable `title: String!`), then that post would evaluate to null, the `posts` list can’t contain nulls and so the list reduces to null.

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This is a companion discussion topic for the original entry at [https://dgraph.io/docs/graphql/api/errors/](https://dgraph.io/docs/graphql/api/errors/)