Marshalling jSON big ints turn into floats to the power

I have this piece of data:

productID, err := products.Insert(map[string]interface{}{
	"Properties": map[string]interface{}{
		strconv.Itoa(propertyNameID): map[string]string{
			"en": "Jeans Jersey",
			"nl": "Broek Jersey",
		},
		strconv.Itoa(propertyColorID): propertyOptionRedID,
	},
	"Type":        productTypeID,
	"Propertyset": propertysetID,
	"Active":      true,
	"EAN13":       "1234567890123"})

All the ***ID variables are of type int. Sadly when I do a normal marshal:

{  
    "Active":true,
    "EAN13":"1234567890123",
    "Properties":{  
        "2286408386526632249":{  
            "en":"Jeans Jersey",
            "nl":"Broek Jersey"
        },
        "4750062295175300168":7.908474319828591e+18
    },
    "Propertyset":8.882218269088507e+18,
    "Type":7.185126253999425e+18
}

... the some ints are transformed into float type to the power.

The Itoa are still just some tests tho, because the marshaller can't do map[int]interface{} (lists with index-values as integers). I just don't understand why the int values get changed to their "display"-value, instead of their pure value.

Update: I tried "Properties" with map[string]int and just one entry. Still the same result

You can marshal an int64 as a string in json to avoid the conversion to a float64 by using the string tag

type T struct {
	Val int64 `json:"val,string"`
}
t := T{Val: math.MaxInt64}
j, _ := json.Marshal(t)
fmt.Println(string(j))
// {"val":"9223372036854775807"}