Error: Expected undefined to be a GraphQL schema.

const { ApolloServer } = require('apollo-server')
const { ApolloGateway } = require('@apollo/gateway')
const { get,post } = require('httpie')
const { readFileSync } = require('fs');

async function getSupergraph() {
let res = await get(`http://localhost:4007/graph`)
return {
supergraphSdl :res.data
};
}

async function startServer() {
const gateway = new ApolloGateway({
experimental_pollInterval: 3000,
experimental_updateSupergraphSdl: async(config) => {
return getSupergraph()
},
__exposeQueryPlanExperimental: false,
})

const server = new ApolloServer({
gateway,
engine: false,
subscriptions: false,
})

server.listen({ port: 4008 }).
then(({ url }) => console.log(`🚀ApolloGateway app running at ${url}`))
.catch(err => {console.error(err)});
}

startServer().catch((err) => {
console.error(err)
process.exit(1)
})

________________
ERROR
________________
Error: Expected undefined to be a GraphQL schema.
at assertSchema (C:\Users\grevant3\Desktop\HUHU\node_modules\graphql\type\schema.js:47:11)
at validateSchema (C:\Users\grevant3\Desktop\HUHU\node_modules\graphql\type\validate.js:42:28)
at assertValidSchema (C:\Users\grevant3\Desktop\HUHU\node_modules\graphql\type\validate.js:66:16)
at assertValidExecutionArguments (C:\Users\grevant3\Desktop\HUHU\node_modules\graphql\execution\execute.js:150:35)
at executeImpl (C:\Users\grevant3\Desktop\HUHU\node_modules\graphql\execution\execute.js:98:3)
at execute (C:\Users\grevant3\Desktop\HUHU\node_modules\graphql\execution\execute.js:60:35)
at generateSchemaHash (C:\Users\grevant3\Desktop\HUHU\node_modules\apollo-server-core\dist\utils\schemaHash.js:15:44)
at ApolloServer.generateSchemaDerivedData (C:\Users\grevant3\Desktop\HUHU\node_modules\apollo-server-core\dist\ApolloServer.js:333:64)
at SchemaManager.schemaDerivedDataProvider (C:\Users\grevant3\Desktop\HUHU\node_modules\apollo-server-core\dist\ApolloServer.js:107:65) at SchemaManager.processSchemaLoadOrUpdateEvent (C:\Users\grevant3\Desktop\HUHU\node_modules\apollo-server-core\dist\utils\schemaManager.js:100:103)

getSupergrap() will return the supergraphSdl string of the managed subgraphs

Start with the ApolloGateway experimental_updateSupergraphSdl configuration and the ApolloServer gateway initialization shown in the reproduction, then run the supplied server example to observe when the schema error occurs. Done means the gateway starts successfully and accepts the polled supergraph SDL without reporting that the schema is undefined.

Written by the indexing model from the issue text.