I have a method intended to generate an email with a users login credentials with a button click.  I used to have it pulling the information off a custom object but am now trying to consolidate with just the standard Contact object, but now in my tests it throws an error in execution.  My code is:
public with sharing class CM_InfoController{

        private static String ORG_WIDE_EMAIL_ADDRESS = 'customerservice@maximusautogroup.com';
               
        public Contact user    {get;set;}
        private static ID orgWideEmailId = [SELECT id FROM OrgWideEmailAddress WHERE address=:ORG_WIDE_EMAIL_ADDRESS].id;
               
    public CM_InfoController(){
    }
   
    public CM_InfoController(ApexPages.StandardController controller){
       user = (Contact)controller.getRecord();
    }   
   
    public PageReference sendEmail(){
        PageReference pageReference = null;
        if(user != null){
                Contact nu = [SELECT Id, Name, Password__c, Email FROM Contact WHERE id=:user.id];
                if(orgWideEmailId != null){
                  // Build new URL
                  Messaging.SingleEmailMessage mail = new Messaging.SingleEmailMessage();
                  String htmlBody = '<h1>MAG User Info Request</h1>' +
                  '<p>Dear ' + nu.Name + ',</p>' +
                  '<p>We have received your request for user credential information in order to access some restricted features of the Maximus Auto Group website.</p>' +
                  '<p>Your Username is: ' + nu.Email + '</p>' +
                  '<p>Your Password is: ' + nu.Password__c + '</p>' +
                  '<p>If you have any questions or concerns regarding this information, please contact one of our associates at 1-800-801-1342 and we will assist you, or reply to this email with any specific questions.</p>' +
                  '<p>Thank you for your business and continued support!</p>' +
                  '<p>MAXIMUS AUTO GROUP</p>';
                                                                      
                   //Build Email
                   mail.setOrgWideEmailAddressId(orgWideEmailId);             
                            String[] toAddresses = new String[]{nu.Email};                                              
                                            mail.setToAddresses(toAddresses);
                                            mail.setSubject('MAG User Access');
                                            mail.setHtmlBody(htmlBody);
                                            mail.setWhatId(user.id);
                                       
                                        Messaging.sendEmail(new Messaging.Email[]{mail},true);
                                                                                                           
                            ApexPages.Message msg = new ApexPages.Message( ApexPages.Severity.INFO, 'Message sent successfully');                  
                            ApexPages.addMessage(msg);                                                                                                 
                                        }
                                        else{
                            ApexPages.Message msg = new ApexPages.Message( ApexPages.Severity.ERROR, 'Org wide email address not found ['+ ORG_WIDE_EMAIL_ADDRESS + ']');                  
                            ApexPages.addMessage(msg);                                                             
                                        }                                                                                                     
                            }   
                                else{
                                ApexPages.Message msg = new ApexPages.Message( ApexPages.Severity.ERROR, 'Email not specified on user to send information.');                  
                                ApexPages.addMessage(msg);                                                             
                                }
                                return(pageReference);
                                }
                                }

And the error I encounter each time is: "First exception on row 0; first error: INVALID_ID_FIELD, Only accounts, assets, campaigns, cases, contracts, opportunities, orders, products, solutions and custom objects are allowed as whatId.: "  which is directed at the "mail.setWhatId(user.id);" line.  However, this was working properly with the custom object I used to have in place so I am unsure why this would suddenly not work when all I did to the code was change the object referenses from my custom one to the Contact one.