Using integer keys for Json (jsoncpp)

I have a C++11 code that takes a couple of Jsons and changes the format. In this procedure I want to construct a Json::Value object that has the following structure:

{ <int value>: [<int value>, <string value>, <string value>, <string value>] }

I do this in the following loop:

for(Json::Value::ArrayIndex ic = 0; ic != yd.size(); ic++){
    Json::Value nn(Json::arrayValue);
    nn.append(yd[ic]["CHANNEL_ID"].asInt   ());
    nn.append(yd[ic]["tech"      ].asString());
    nn.append(yd[ic]["file"      ].asString());
    nn.append(yd[ic]["data"      ].asString());
    Json::Value mm;
    mm[yd[ic]["CHANNEL_ID"].asInt()] = nn;
    afnew.append(mm);
}

Here yd is the json that I read from, and I want to append everything to afnew. The node CHANNEL_ID is a large number (I may even consider using unsigned longs here, but anyway). The output is ok BUT it appends a lot of nulls in front of the actual array. Essentially, if the channel number is 500, it will append 499 nulls in front of it. If I convert the channel number to string and use this as key for the mm json, it works (also forcing the mm to Json::objectValue). Is there a way around using strings here as keys, or will the integer values always be interpreted as vector indices?

Thank you for any advice and regards!

c.