Error creating custom field on photo

I am trying to attach a custom string to a photo which a user uploads on the server. I use the following method:

Map<String, Object> data = new HashMap<String, Object>();
data.put("photo", f);
data.put("custom_fields", "{\"description\":" + "\"" + desc + "\"}");

'desc' is the string which i want to attach as 'description'.

I get the response as: {"message":"Failed to set custom_fields: 745: unexpected token at '%7B%22description%22%3A%22Moon%22%7D'","status":"fail","code":400}

(Moon was just a trial String)

Any help would be great. :)

I solved it myself:

It should be :

data.put("custom_string_description","Your String to Attach");

where 'description' is the name of string custom field. 'string' is the type of custom field you want to create.