why tag name invalid

[wangxingcong]

struct example

type Range struct {
	StarCell []int `mapstructure:"startCell"`
	EndCell  []int `mapstructure:"endCell"`
}

type Table struct {
	Range []Range `mapstructure:"ranges"`
}

func Test_Table(t *testing.T) {
	table := Table{
		Range: []Range{
			{StarCell: []int{1, 2}, EndCell: []int{2, 3}},
			{StarCell: []int{3, 4}, EndCell: []int{4, 5}},
		},
	}
	var m map[string]interface{}
	err := mapstructure.Decode(table, &m)
	assert.NoError(t, err)
	marshal, err := json.Marshal(m)
	assert.NoError(t, err)
	t.Errorf(string(marshal))
}

i want got Range.StartCell key is lower start, but i got StartCell

{"ranges":[{"StarCell":[1,2],"EndCell":[2,3]},{"StarCell":[3,4],"EndCell":[4,5]}]}

why tag does invalid