Mandrill API mis-reports bad formats - google-analytics

Question

As specified in the documentation, I have to send arrays for google_analytics_domains and google_analytics_campaign fields.
http://mandrillapp.com/api/docs/messages.html#method=send
But mandrill generates a link with a wrong utm_campaign (Google analytics doesn't like the [0] part) :
utm_campaign[0]=test_campaign
I tried to send strings instead of arrays but the mandrallapi kicks me out (Please enter an array)... Did i miss something ? Thanks for your help !
Here is a request example with mandrillapp:
{
"key":"apikey",
"message":{
"html":"example html",
"text":"example text",
"subject":"example subject",
"from_email":"from_email#example.com",
"from_name":"example from_name",
"to":[
{
"email":"email#example.com",
"name":"example name"
}
],
"headers":{
"...":"..."
},
"track_opens":true,
"track_clicks":true,
"auto_text":true,
"url_strip_qs":true,
"bcc_address":"bcc_address#example.com",
"merge":true,
"global_merge_vars":[
{
"name":"example name",
"content":"example content"
}
],
"merge_vars":[
{
"rcpt":"rcpt#example.com",
"vars":[
{
"name":"example name",
"content":"example content"
}
]
}
],
"tags":[
"example tags[]"
],
"google_analytics_domains":[
"..."
],
"google_analytics_campaign":[
"..."
],
"metadata":[
"..."
],
"attachments":[
{
"type":"example type",
"name":"example name",
"content":"example content"
}
]
}
}