When String is passed to add method of List<BigDecimal>, exec-model fails while non-exec-model allows

rule "rule_add_list"
    no-loop true
  when
        $fact : Fact( str == "ADD", $val : "100")
  then
        modify($fact) {
                bdList.add($val);
        }
end

The method add(BigDecimal) in the type List<BigDecimal> is not applicable for the arguments (String)