Whilst uploading JSON, adding timestamp removes other data

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I'm trying to add a timestamp with some other data.

If I run:

Firebase.pushJSON(fbdo, "/localWeather", "{\"temp\":\"" + temp + "\",\"humidity\":\"" + humidity + "\"}")

The data is uploaded successfully:

However, when I try to add a timestamp to the JSON only the timestamp is uploaded:

Firebase.pushJSON(fbdo, "/localWeather", "{\"temp\":\"" + temp + "\",\"humidity\":\"" + humidity + "\",\".sv\": \"timestamp\"" + "}")

My expected outcome is something like this for each entry: { "x":{ "temp":"23c", "humidity":"50%", "timestamp":"[unix timestamp]" } }

My question I guess is what am I doing wrong here?