Unexpected Behavior in Schema Decoding: No Error for Undefined Value

import { Schema } from "@effect/schema"

const schema = Schema.Struct({
  a: Schema.Unknown
})

// @ts-ignore
Schema.decodeSync(schema)({}) // type error. expected

const result = Schema.decodeUnknownSync(schema)({})
console.log(result) // { a: undefined } ?! I would expect a decoding error.