Missouri was International Shoe Corporation's principal place of business, but the company employed between 11 and 13 salespersons in the state of Washington who exhibited samples and solicited orders for shoes from prospective buyers in Washington. The state of Washington assessed the company for contributions to a state unemployment fund. The state served the assessment on one of International Shoe Corporation's sales representatives in Washington and sent a copy by registered mail to the company's Missouri headquarters. International Shoe's representative challenged the assessment on numerous grounds, arguing that the state had not properly served the corporation. Is the corporation's defense valid? Why or why not?