I've reproduced the problem also in 3.7 branch.

```
import asyncio

loop = asyncio.get_event_loop()


async def consumer():
    while True:
        await asyncio.sleep(0)
        message = yield
        print('received', message)


async def amain():
    agenerator = consumer()
    await agenerator.asend(None)

    fa = asyncio.create_task(agenerator.asend('A'))
    fb = asyncio.create_task(agenerator.asend('B'))
    await fa
    await fb


loop.run_until_complete(amain())
```

Output:
```
received A
received None
```

If the line `await asyncio.sleep(0)` is omitted the output is ok:
```
received A
received B
```