geojson Decode()

[lc-dmx]

fc := `{"type":"FeatureCollection","features":[{"type":"Feature","geometry":{"type":"Polygon","coordinates":[[[122.993197,41.117725],[122.999399,41.115696],[122.99573,41.109516],[122.987146,41.106994],[122.984775,41.107699],[122.990687,41.117878],[122.993197,41.117725]]]},"properties":{}}]}`
_, err := geoencoding.Decode([]byte(fc), geoencoding.GeoJSON)
fmt.Println(err)

report error: geojson: not a feature: type=FeatureCollection

I thought the correct should be
if strings.Contains(string(s[9:26]), "FeatureCollection") {
rather than
if strings.Contains(string(s[9:20]), "FeatureCollection") {

[lc-dmx]

Can someone fix it quickly? thanks a lot