work with apollo cli in postgraphile
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i run postgraphile with this:

```js
import cors from 'cors';
import express from 'express';
import bodyParser from 'body-parser';
import { postgraphileConfig } from './postGraphile';
import dotenv from 'dotenv';
dotenv.config();

const app = express();

const { PORT } = process.env;

app.use(cors());
app.use(bodyParser.json());
app.use(bodyParser.urlencoded({ extended: false }));
app.use(postgraphileConfig);

app.listen(PORT, () => console.log(`Server running on port ${PORT}`));
```

and

```js
const postgraphileConfig = postgraphile(ROOT_PG_POOL, 'sample', {
  appendPlugins: [
    PgSimplifyInflectorPlugin,
    ConnectionFilterPlugin,
    ManyToManyPluign,
  ],
  retryOnInitFail: true,
  watchPg: true,
  graphiql: true,
  enhanceGraphiql: true,
  pgDefaultRole: 'sample_anonymous',
  jwtSecret: JWT_KEY || 'sample secret',
  jwtPgTypeIdentifier: 'sample.jwt',
  allowExplain: true,
});
```

but when i run 

```
npx apollo schema:download --endpoint="http://localhost:6060/graphiql"
```

post is correct

it give me this error:

```
 ✔ Loading Apollo Project
  ✖ Saving schema to schema.json
    → ServerParseError: Unexpected end of JSON input
```

it work good at palyground.
do u know where is problem?

