How to use sum only on number field and group by string field, while having both string and number fields in select statement in InfluxDB's FluxQL?

This FLUX Query is resulting in error : sum: unsupported aggregate column type string from(bucket: "Bucket") |> range(start: v.timeRangeStart, stop: v.timeRangeStop) |> filter(fn: (r) => r["_measurement"] == "measurement") |> filter(fn: (r) => r["_field"] == "string_field" or r["_field"] == "number_field") |> group(columns:["string_field"]) |> window(every: 3mo) |> sum(columns:["number_field"]) |> map(fn: (r) =>({ r with _time: r._start})) |> window(every: inf) |> yield()