Not receiving code parameter when i install app on development store

[workscaler]

Hi @Anonymous @Shopify 
I am working on public shopify app which is not listed yet.
I have written my application in nodejs. When i redirect from my app to /admin/oauth/authorize endpoint to install app, it successfully redirects me to my redirect_uri and i receive hmac, code and shop in query parameter.
But when i logged into my partner account => go to my apps section => click on my app => more actions => test on development store

It redirects me to install app popup, i clicked on install unlisted app (as my app is not listed yet), it redirect to my redirect_uri but code parameter is missing in query parameter. shop and hmac is there but not code.