angular2 formBuilder group async validation

I'm trying to implement an async validator with no success...

My component creates a form with :

this.personForm = this._frmBldr.group({
  lastname:  [ '', Validators.compose([Validators.required, Validators.minLength(2) ]) ],
  firstname: [ '', Validators.compose([Validators.required, Validators.minLength(2) ]) ],
  birthdate: [ '', Validators.compose([ Validators.required, DateValidators.checkIsNotInTheFuture ]) ],
  driverLicenceDate: [ '', Validators.compose([ Validators.required, DateValidators.checkIsNotInTheFuture ]), this.asyncValidationLicenceDate.bind(this) ],
}, {
  asyncValidator: this.validateBusiness.bind(this),
  validator: this.validateDriverLicenseOlderThanBirthdate,
});

My validation method

validateBusiness(group: FormGroup) {
  console.log('validateBusiness')
  return this._frmService
    .validateForm(group.value)
    .map((validationResponse: IValidationResponse) => {
      if (validationResponse) {
        validationResponse.validations.forEach( (validationError: IValidationErrorDescription) => {
                        let errorMsg = validationError.display;
                        let errorCode = validationError.code;
                        validationError.fields.forEach( (fieldName: string) => {
                            console.log(fieldName);
                            let control = this.personForm.controls[fieldName];
                            let existingErrors = control.errors || {};
                            existingErrors[errorCode] = errorMsg;
                            control.setErrors(existingErrors);
                        });
                    });
                }
            });
    }

All validations are called successfuly, except the validateBusiness method (in the extra.asyncValidator param of the formbuilder.group) which is never called... Can somebody tell me what I'm doing wrong ?

Tx