列索引1上的扫描错误：不支持的扫描，将驱动程序存储。值类型<nil>转换为* string类型

I have a column in postgres which is blank initially

var dtime sql.NullString

err = rows.Scan(&dtime)

if dtime.Valid {
    dtimeString = dtime.String
} else {
    dtime = sql.NullString{}
}

rows are are a result set of a sql query from my table.

select dtime from mytable;  // returns 5 rows.

But I keep getting

Scan error on column index 1: unsupported Scan, storing driver.Value type <nil> into type *string.