INVALID_ID_FIELD, All targetObjectIds for an email must be of the same type error

Question

I am getting the below error when the below trigger is triggered. Can anyone tell me how to resolve this error -

Error -

Error: Invalid Data. Review all error messages below to correct your data. Apex trigger EmailService caused an unexpected exception, contact your administrator: EmailService: execution of AfterInsert caused by: System.EmailException: SendEmail failed. First exception on row 0; first error: INVALID_ID_FIELD, All targetObjectIds for an email must be of the same type.: []: Trigger.EmailService: line 20, column 1

Code -

trigger EmailService on Task__c (after insert) {
    EmailTemplate et=[Select id from EmailTemplate where name='Multiple Tasks'];
    List<Messaging.MassEmailMessage> mails = new List<Messaging.MassEmailMessage>();

    for(Task__c t : Trigger.new)
    {
        if(t.Project_Approval__c =='Approved' &&t.Status__c == 'Open' ){
            List<Id> lstIds= new List<Id>();
            lstIds.add(t.Task_Executioner__c);
            lstIds.add(t.Task_Executioner_1__c);
            lstIds.add(t.Task_Executioner_2__c);

            Messaging.MassEmailMessage mail = new Messaging.MassEmailMessage();
            mail.setTargetObjectIds(lstIds);
            mail.setSenderDisplayName('Charan Tej');
            mail.setTemplateId(et.id);
            mails.add(mail);
       }
    }
    Messaging.sendEmail(mails);
}

Answer 1

From the docs on method setTargetObjectIds of MassEmailMessage:

The objects must be of the same type (all contacts, all leads, or all users).

You are likely trying to add mixed IDs to the collection of IDs that you then pass to this method. Please verify that the fields Task_Executioner__c, Task_Executioner_1__c, and Task_Executioner_2__c are IDs, and that they are all either contacts, users, or leads.