Why Boolean is not accepting null value?

Question

    public Map<String, BigDecimal> getData(Set<String> ids, Boolean flag) {
    Map<String, BigDecimal> data = new HashMap<>();
    if(flag == null){
        try {
            data = *code logic*
        } catch (RuntimeException e) {
            log.error("Error occurred while fetching data : {}", e.getMessage());
        }
    }
    else{
        try {
            data = *code logic*;
        } catch (RuntimeException e) {
            log.error("Error occurred while fetching data : {}", e.getMessage());
        }
    }
    return data;

Whenever the value of flag is passed as null I am getting this error :

Failed to convert value of type 'java.lang.String' to required type 'java.lang.Boolean'; nested exception is java.lang.IllegalArgumentException: Invalid boolean value [null]

Why is this happening? Isn't the wrapper class Boolean supposed to hold values true, false and null?