kotlin エラー「error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch:」の解決方法

fun main() {

    val m = mapOf('a' to 1, 'b' to 2, 'c' to 3)

    println(m) // {a=1, b=2, c=3}

    println(m['a']) // 1

    m['b'] = 5

}

test.kt:9:5: error: unresolved reference. None of the following candidates is applicable because of receiver type mismatch:
public inline operator fun kotlin.text.StringBuilder /* = java.lang.StringBuilder */.set(index: Int, value: Char): Unit defined in kotlin.text
    m['b'] = 5
    ^
test.kt:9:6: error: no set method providing array access
    m['b'] = 5
     ^

fun main() {

    val m = mutableMapOf('a' to 1, 'b' to 2, 'c' to 3)

    println(m) // {a=1, b=2, c=3}

    println(m['a']) // 1

    m['b'] = 5

    println(m['b']) // 5

}