exception when use MutableCollection

[4o4E]

use MutableCollection<User>

import com.github.jershell.kbson.KBson
import kotlinx.serialization.Serializable
import org.bson.BsonArray
import org.bson.BsonDocument
import org.bson.BsonElement
import org.bson.BsonString

@Serializable
data class Container(
    val users: MutableCollection<User>
)

@Serializable
data class User(val name: String)

fun main() {
    val bsonDocument = BsonDocument(
        mutableListOf(
            BsonElement("users", BsonArray(mutableListOf(BsonDocument("name", BsonString("a"))))),
        )
    )
    println(KBson.default.parse(Container.serializer(), bsonDocument))
}

exception

Exception in thread "main" org.bson.BsonInvalidOperationException: readStartDocument can only be called when CurrentBSONType is DOCUMENT, not when CurrentBSONType is ARRAY.
	at org.bson.AbstractBsonReader.verifyBSONType(AbstractBsonReader.java:689)
	at org.bson.AbstractBsonReader.checkPreconditions(AbstractBsonReader.java:721)
	at org.bson.AbstractBsonReader.readStartDocument(AbstractBsonReader.java:449)
	at com.github.jershell.kbson.FlexibleDecoder.beginStructure(BsonFlexibleDecoder.kt:51)
	at kotlinx.serialization.internal.AbstractPolymorphicSerializer.deserialize(AbstractPolymorphicSerializer.kt:121)
	at kotlinx.serialization.encoding.Decoder$DefaultImpls.decodeSerializableValue(Decoding.kt:257)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableValue(AbstractDecoder.kt:16)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableValue(AbstractDecoder.kt:43)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableElement(AbstractDecoder.kt:70)
	at Container$$serializer.deserialize(a.kt:8)
	at Container$$serializer.deserialize(a.kt:8)
	at kotlinx.serialization.encoding.Decoder$DefaultImpls.decodeSerializableValue(Decoding.kt:257)
	at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableValue(AbstractDecoder.kt:16)
	at com.github.jershell.kbson.KBson.parse(KBson.kt:22)
	at AKt.main(a.kt:22)
	at AKt.main(a.kt)

if use List<User>, no exception

@Serializable
data class Container(
    val users: List<User>
)

@Serializable
data class User(val name: String)

fun main() {
    val bsonDocument = BsonDocument(
        mutableListOf(
            BsonElement("users", BsonArray(mutableListOf(BsonDocument("name", BsonString("a"))))),
        )
    )
    println(KBson.default.parse(Container.serializer(), bsonDocument))
}