You work as a Network Administrator for Net Perfect Inc. The company's intranet Web site netperfect.com uses a digital certificate issued by the company's Enterprise Certificate Authority. The Enterprise Certificate Authority is located on a server named certificates.netperfect.com. Kate, a user, complains that she is unable to access the secure Web site on her computer using Internet Explorer. An error message appears stating that the digital certificate is not from a trusted source. What is the most likely cause?