Cannot return null for non-nullable field when throwing error

[erikmellum]

Describe the issue
When I throw an error in a mutation, the custom error is being discarded in favor of "cannot return null for non-nullable field". I suspect I am doing something wrong, but I do not see in the documentation what the problem is. I am hoping that some guidance can be provided, with hopes that this will also help someone else in the future.

@Resolver(() => TimeOffRequest)
export class TimeOffRequestCustomResolver implements ResolverInterface<TimeOffRequest> {
  @Authorized(agentEdit, supervisorEdit, adminEdit)
  @Mutation(() => TimeOffRequest)
  async createTimeOffRequest(
    @Ctx() { timeOffRequestService, request }: Context,
    @Arg('data', () => CreateTimeOffRequestInput) data: CreateTimeOffRequestInput
  ): Promise<TimeOffRequest> {
    const { accountId, userId } = getAccountIdAndUserIdFromToken(request)
    throw new GraphQLError('Invalid argument value', {
      extensions: {
        code: 'BAD_USER_INPUT',
      },
    })
}

When I run this, the graphql error formatter is seeing: 'Cannot return null for non-nullable field Mutation.createTimeOffRequest.'