kotlinx decodeFromString to 'ArrayList' vs 'CustomClass extending ArrayList'

Question

Why does this work:

val string = "[1, 2, 3]"
val result: ArrayList<Int> = json.decodeFromString(string)

but this doesn't:

@Serializable
class MyClass : ArrayList<Int>()

val string = "[1, 2, 3]"
val myClass: MyClass = json.decodeFromString("string")

throws this error:

Expected start of the object '{', but had 'EOF' instead

MyClass is also an ArrayList so I'm unsure why it doesn't work