Arithmetic operation with String coercion in constraint fails in executable-model

rule R1
dialect "mvel"
when
  StrBdHolder($str: str, bd == $str * -1)
then
end

The operator * is undefined for the argument type(s) String, int

public enum LambdaExtractorA01091D610A021769F6F9F5A86246BC8 implements org.drools.model.functions.Function1<java.lang.String, java.lang.Object>, org.drools.model.functions.HashedExpression {
   ...
    @Override()
    public java.lang.Object apply(java.lang.String $str) {
        return $str * -1;
    }
}