accumulate sum() of BigDecimal values incorrectly returns a value of type Double.

Issue

accumulate sum() of BigDecimal values incorrectly returns a value of type Double in MVEL dialect and non-executable model environment.
For example, when I run a rule which calculates sum of BigDecimal values in such environment using (*1) and (*2),

(*1) Example rule

    :
    :
dialect "mvel"

rule "Rule1"
    when
        $sum : BigDecimal() from accumulate( Charge( $amount : amount != null ), sum($amount) )
    then
        System.out.println("***** Action of Rule1: " + $sum + " (" + $sum.getClass().getName() + ")");
end

(*2) Java program which runs the rule (*1)

        :
        :
    for (int i = 1; i <= 10; i++) {
        Charge charge = new Charge(new BigDecimal(i));
        System.out.println("input: " + charge);
        kSession.insert(charge);
    }
    kSession.fireAllRules();
        :
        :

the resulting value will not be of type BigDecimal but be of type Double incorrectly like (*3) below.

(*3) Result

***** Action of Rule1: 55.0 (java.lang.Double)