failure to execute limelm.pkey.deactivateAnswered

Hi,

I'm trying to implement something in the API:

first I get the key details using limelm.pkey.find. I get this response:

{"pkeys":{"total":1,"pkey":[{"id":6257051,"key":"####-####-####-####-####-####-####","acts":1,"acts_used":1}]},"stat":"ok"}

afterwards I try to deactivate that key. for that I'm using 6257051 as my “act_id”. However, the response I'm getting is 

{"stat":"fail","code":128,"message":"The activation doesn't exist or you don't have access to it."}

I'm doing that with Postman. These are the headers & the body of the deactivation request:

//Content-Type:application/x-www-form-urlencoded

api_key: ****************************************
format:json
nojsoncallback:1
method:limelm.pkey.deactivate
act_id:6257051
version_id:5644

What am I doing wrong?