Kong: failed to run body_filter_by_lua body must be a string

I try to proxy FastAPI application behind kong everything is fine.

but for openapi.json I got 502 at Kong and log is

failed to run body_filter_by_lua*: /usr/local/share/lua/5.1/kong/init.lua:326: body must be a string

I run on Kubecluster using nginx ingress but everything is fine if i access to fastapi directly is good can access openapi.json

but when behind kong kong error cann't proxy for openapi.json