asyncpg: asyncpg.exceptions._base.InterfaceError: cannot perform operation: another operation is in progress

Hi, here is my code ,please help resovle the problem.thanks a lot.

from sanic import Sanic
from sanic.response import json
import asyncpg
import asyncio
import uvloop
loop = uvloop.new_event_loop()
app = Sanic()
app.debug = True

async def initdb_pool():
    dbdict = {"database":"mqtt","user":"mqtt","password":"mqtt123",
            "host":"192.168.25.100","port":5433}
    return await asyncpg.create_pool(**dbdict)

@app.route("/iot/v1.0/app/auth/<user:[A-z0-9]\w+>/<pwd:[A-z0-9]\w+>/")
async def applogin(request,user,pwd):
    async with engine.acquire() as connection:
        #async with connection.transaction():
        stmt   = await connection.prepare('select key from user_manager_appuser where uname = $1 or email = $2 or phone = $3')
        result =  await stmt.fetchval(user,user,user)
        #    result = await connection.fetchval('select key from user_manager_appuser where uname = $1 or email = $2 or phone = $3',
        #                                        user,user,user)
        if not result:
            return json({'ok':False,'err':'Pwd error'})
        print("result is ",result)
        return json({'ok':True,'data':str(result)})

if __name__ == "__main__":
    engine = loop.run_until_complete(initdb_pool())
    app.run(host="0.0.0.0",port=8000,debug=True)

 curl "http://127.0.0.1:8000/iot/v1.0/app/auth/abc/123456/"
Error: cannot perform operation: another operation is in progress
Exception: Traceback (most recent call last):
  File "/media/yjdwbj/E/py360dev/lib/python3.6/site-packages/sanic/sanic.py", line 194, in handle_request
    response = await response
  File "/opt/python360/lib/python3.6/asyncio/coroutines.py", line 128, in throw
    return self.gen.throw(type, value, traceback)
  File "main.py", line 32, in applogin
    return json({'ok':True,'data':str(result)})
  File "/opt/python360/lib/python3.6/asyncio/coroutines.py", line 109, in __next__
    return self.gen.send(None)
  File "/media/yjdwbj/E/py360dev/lib/python3.6/site-packages/asyncpg/pool.py", line 252, in __aexit__
    await self.pool.release(con)
  File "/opt/python360/lib/python3.6/asyncio/coroutines.py", line 109, in __next__
    return self.gen.send(None)
  File "/media/yjdwbj/E/py360dev/lib/python3.6/site-packages/asyncpg/pool.py", line 184, in release
    await connection.reset()
  File "/opt/python360/lib/python3.6/asyncio/coroutines.py", line 109, in __next__
    return self.gen.send(None)
  File "/media/yjdwbj/E/py360dev/lib/python3.6/site-packages/asyncpg/connection.py", line 411, in reset
    ''')
  File "/opt/python360/lib/python3.6/asyncio/coroutines.py", line 109, in __next__
    return self.gen.send(None)
  File "/media/yjdwbj/E/py360dev/lib/python3.6/site-packages/asyncpg/connection.py", line 170, in execute
    return await self._protocol.query(query, timeout)
  File "asyncpg/protocol/protocol.pyx", line 247, in query (asyncpg/protocol/protocol.c:51830)
  File "asyncpg/protocol/protocol.pyx", line 352, in asyncpg.protocol.protocol.BaseProtocol._ensure_clear_state (asyncpg/protocol/protocol.c:54136)
asyncpg.exceptions._base.InterfaceError: cannot perform operation: another operation is in progress