GraphQL Type Error for Union Type when trying to run a query ([Error: Unknown type ...])

I have set up a graphql schema where one of the fields ('items') is a list of objects which are a Union Type i.e. the list items can be one of two object types (in this case they are called 'packageOfferItem' and 'tileItem').

var searchResultItemType = new graphql.GraphQLUnionType({
  name: 'SearchResultItems',
  types: [ packageOfferItem, tileItem ],
  resolveType: function (value) {
    if (value.type === 'packageOffer') {
      return packageOfferItem;
    } else if (value.type === 'tile') {
      return tileItem;
    } else {
      return null;
    }
  }
});

var searchResultItemsType = new graphql.GraphQLList(searchResultItemType);

The items field is then of type searchResultItemsType.

In my tests I have a query which requests data from items filed using the notation:

{ 
  items {
     ...on packageOfferItem {
       id
    }
  }
}

In my test I get an error [Error: Unknown type "packageOfferItem".] message: 'Unknown type "packageOfferItem".' } ]

Where is this error coming from?? packageOfferItem is a graphql NamedType so I don't understand why there is an error in the query response.

Introspection of the schema does not throw any errors so the schema itself is syntactically valid.