数组型interface赋值报错问题请教

```golang package main type Entity interface { Say() string } type Cat struct{} func (c Cat) Say() string { return "我是一只猫" } type Union struct { Mem []Entity } func main() { var cats []Cat cats = append(cats, Cat{}) cats = append(cats, Cat{}) cats = append(cats, Cat{}) u := Union{ Mem: cats, } } ``` 如上代码，声明一个名为```Entity```的```interface```，包含一个方法```Say() string```，再声明一个名为```Cat```的```struct```实现该方法，我把```Union```的```Mem```属性声明为```[]Entity```类型，但是对该字段赋值时会报错如下： ```shell : cannot use cats (type []Cat) as type []Entity in field value ``` 求大佬指教，该怎么调整？？？