尝试在函数中返回多种类型的接口时，语法类型脚本错误

package main

import "fmt"

type Shape interface {
    Area() float64
}

type Circle struct {
    Radius float64
}

func (c Circle) Area() float64 {
    return 3.14 * c.Radius * c.Radius
}

type Rectangle struct {
    Width  float64
    Height float64
}

func (r Rectangle) Area() float64 {
    return r.Width * r.Height
}

func GetShape() interface{} {
    if someCondition {
        return Circle{Radius: 5}
    } else {
        return Rectangle{Width: 4, Height: 6}
    }
}

func main() {
    shape := GetShape()

    switch shape := shape.(type) {
    case Circle:
        fmt.Println("Circle area:", shape.Area())
    case Rectangle:
        fmt.Println("Rectangle area:", shape.Area())
    default:
        fmt.Println("Unknown shape")
    }
}