interface Person {
  id: ID!
  name: String
}

type A implements Person {
  id: ID!
  name: String
}

type B implements Person {
  id: ID!
  name: String
}

query {
  Person {
    name
  }
}

Abstract type \"Person\" must resolve to an Object type at runtime for field \"Query.Person\" with value { name: \"A Person\", FRAGMENT_TYPE: null }, received \"null\". Either the \"Person\" type should provide a \"resolveType\" function or each possible type should provide an \"isTypeOf\" function.

GraphQLError: Abstract type "Person" must resolve to an Object type at runtime for field "Query.Person" with value { name: "Giles Dring", FRAGMENT_TYPE: null }, received "null". Either the "Person" type should provide a "resolveType" function or each possible type should provide an "isTypeOf" function.
    at ensureValidRuntimeType (/Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:687:11)
    at completeAbstractValue (/Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:680:42)
    at completeValue (/Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:604:12)
    at completeValueCatchingError (/Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:514:19)
    at /Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:636:25
    at from (<anonymous>)
    at completeListValue (/Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:632:49)
    at completeValue (/Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:592:12)
    at /Users/gilesdring/src/dringtech/eccoo/graph/api/node_modules/graphql/execution/execute.js:511:16
    at processTicksAndRejections (internal/process/task_queues.js:97:5)