Troubleshooting Guide: Fixing Cannot Return Null for Non-Nullable Field OfferUp Errors

This guide provides step-by-step instructions for troubleshooting and fixing "Cannot Return Null for Non-Nullable Field" errors in OfferUp. This error occurs when the GraphQL API returns a null value for a field that is designated as non-nullable in the schema definition.

Table of Contents

  1. Understanding the Error
  2. Step-by-Step Troubleshooting Guide
  3. Check the Schema Definition
  4. Verify the Resolver Function
  5. Debug the Data Source
  6. Examine the GraphQL Query
  7. FAQ

Understanding the Error

The "Cannot Return Null for Non-Nullable Field" error typically occurs when a GraphQL query requests a non-nullable field, but the resolver function returns a null value. This is problematic because non-nullable fields are required to return a value, and GraphQL will not proceed with the query execution.

To resolve this issue, you need to identify why the resolver function is returning a null value and fix the issue.

Step-by-Step Troubleshooting Guide

Check the Schema Definition

The first step in troubleshooting the error is to examine your schema definition. Make sure that the field in question is correctly defined as non-nullable (using the ! symbol).


type Offer {
  id: ID!
  title: String!
  description: String

In this example, both id and title are non-nullable fields, whereas description is nullable.

If the field should be nullable, update your schema definition accordingly.

Verify the Resolver Function

The next step is to check the resolver function responsible for returning the data for the non-nullable field. Make sure that the function always returns a value for that field.


const resolvers = {
  Offer: {
    id: (parent) =>,
    title: (parent) => parent.title,
    description: (parent) => parent.description || 'No description provided',

In this example, the id and title resolvers are expected to always return a value, while the description resolver provides a default value in case the parent object does not have a description.

If necessary, update your resolver function to ensure that it always returns a value for non-nullable fields.

Debug the Data Source

If your resolver function relies on a data source (such as a database or API) to fetch the data, make sure that the data source is functioning correctly and returning the expected data.

You can use debugging tools or logging statements to help identify any issues with your data source.


const resolvers = {
  Offer: {
    id: (parent) => {
      console.log('Fetching ID from parent:', parent);
    title: (parent) => {
      console.log('Fetching title from parent:', parent);
      return parent.title;
    description: (parent) => {
      console.log('Fetching description from parent:', parent);
      return parent.description || 'No description provided';

In this example, we have added console.log statements to the resolver functions to help debug the data source and identify any issues with the parent object.

Examine the GraphQL Query

Lastly, examine the GraphQL query that is causing the error to ensure that it is correctly requesting the non-nullable field.


query {
  offer(id: "1") {

In this example, the query is correctly requesting the non-nullable fields id and title. If your query is not properly requesting the non-nullable field, update it accordingly.


1. What is a non-nullable field in GraphQL?

A non-nullable field is a field in a GraphQL schema that must always return a value. It cannot return null. Non-nullable fields are defined with the ! symbol in the schema.

2. How do I make a field nullable in my GraphQL schema?

To make a field nullable, simply remove the ! symbol from the field definition in your schema.

3. Can I use default values for non-nullable fields in GraphQL?

Yes, you can provide default values for non-nullable fields in your resolver functions to ensure that a value is always returned.

4. How do I debug my resolver functions?

You can use debugging tools, logging statements, or other techniques to debug your resolver functions and identify any issues with the data they return.

5. How can I ensure my data source is functioning correctly?

You can use monitoring, logging, and testing tools to ensure that your data source is functioning correctly and returning the expected data for your GraphQL API.

Related: Understanding GraphQL Errors
Related: Debugging GraphQL APIs

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