title: SyntaxError: Unexpected token < in JSON at position 0 when testing in Graphiql
body: <p>I am learning GraphQL and am new to the technology. I am unable to figure out the cause for this syntax error. When I am testing it on graphiql it throws an unexpected token syntax error</p>
  
  <p>Here is my server.js:</p>
  
  <pre><code>const express = require("express");
  const graphqlHTTP = require("express-graphql");
  const schema = require("./schema");
  
  const app = express();
  app.get(
    "/graphql",
    graphqlHTTP({
      schema: schema,
      graphiql: true
    })
  );
  
  app.listen(4000, () =&gt; {
    console.log("Server listening to port 4000...");
  });
  </code></pre>
  
  <p>Here is my schema:</p>
  
  <pre><code>const {
    GraphQLObjectType,
    GraphQLString,
    GraphQLInt,
    GraphQLSchema,
    GraphQLList,
    GraphQLNotNull
  } = require("graphql");
  
  // HARD CODED DATA
  const customers = [
    { id: "1", name: "John Doe", email: "jdoe@gmail.com", age: 35 },
    { id: "2", name: "Kelly James", email: "kellyjames@gmail.com", age: 28 },
    { id: "3", name: "Skinny Pete", email: "skinnypete@gmail.com", age: 31 }
  ];
  
  // CUSTOMER TYPE
  const CustomerType = new GraphQLObjectType({
    name: "Customer",
    fields: () =&gt; ({
      id: { type: GraphQLString },
      name: { type: GraphQLString },
      email: { type: GraphQLString },
      age: { type: GraphQLInt }
    })
  });
  
  // ROOT QUERY
  const RootQuery = new GraphQLObjectType({
    name: "RootQueryType",
    fields: {
      customer: {
        type: CustomerType,
        args: {
          id: { type: GraphQLString }
        },
        resolve(parentValue, args) {
          for (let i = 0; i &lt; customers.length; i++) {
            if (customers[i].id == args.id) {
              return customers[i];
            }
          }
        }
      },
      customers: {
        type: new GraphQLList(CustomerType),
        resolve(parentValue, args) {
          return customers;
        }
      }
    }
  });
  
  module.exports = new GraphQLSchema({
    query: RootQuery
  });
  </code></pre>
  
  <p>Can someone point me in the right direction? I can't figure out the problem here?</p>