Migration token OAuth 1 to Oauth 2

Good Afternoon 

I triyed to update my OAuth 1 token to OAuth 2 through this HTTP request  :

https://api.dropboxapi.com/2/auth/token/from_oauth1

But the response looks like this : 

{"error_summary": "invalid_oauth1_token_info/.", "error": {".tag": "invalid_oauth1_token_info"}}

This kind of error references to the token information ( key:secret ).

The parameter's data  is :

$auth = $key.':'.$secret

array("oauth1_token"=>$auth,"oauth1_token_secret"=>$secret);

But the values was correct !( I think so...)

Whats going on ?