API disabled in ngx.timer.at

[FMLS]

hi, I got a Wicked problem: i must open session in body_filter phase, so i must use <ngx.timer.at api> to wapper <session.start_session>, but i got this error and i check the doc of openresty, maybe ngx.var.VARIABLE can't be used in ngx.timer.at context, what should i do? please help me

[error] 3979#0: *363 lua entry thread aborted: runtime error: ...ty@1.13.6.2/1.13.6.2/openresty/lualib/resty/core/var.lua:73: API disabled in the current context
stack traceback:
coroutine 0:
[C]: in function 'error'
...ty@1.13.6.2/1.13.6.2/openresty/lualib/resty/core/var.lua:73: in function '__index'
./src/libs/third/resty/session.lua:174: in function 'init'
./src/libs/third/resty/session.lua:215: in function 'new'
./src/libs/third/resty/session.lua:275: in function 'open'
./src/libs/third/resty/session.lua:303: in function 'start_session'