I'm trying to get a list of contact ids out of a list of campaignmembers, to do this i first build the list of campaign members, then attempt to put the contactid into a set.  I'm running into the error at this point and can't figure out out to fix it.

global with sharing class CampaignROI_Reps{

    public static list<CampaignMember> cList;
    public static set<id> contacts;

    public AggregateResult SalesResultsPost30 {get;set;}

    public CampaignROI_Reps(){
        
        //get distribution aggregate data
        cList = [SELECT id, contactid, campaignid, rep_name__c, firm__c,
                     reit_sales__c, reit_tickets__c, calls__c, meetings__c, pipeline__c, 
                     territory__c
                 FROM CampaignMember
                 WHERE Campaignid =:ApexPages.currentPage().getParameters().get('id')];
       
       if(clist.size() > 0){
           for(campaignmember c:clist){
               if(c.contactid != null){
                   contacts.add(c.contactid);
               }
           }
       }
       
       SalesResultsPost30 = [SELECT SUM(Gross_Amount__c) total1
                       FROM Transaction__c 
                       WHERE Rep__c in: contacts AND 
                           trade_date__c >: date.valueof(ApexPages.currentPage().getParameters().get('enddate')) AND
                           trade_date__c <: date.valueof(ApexPages.currentPage().getParameters().get('enddate')) + 30];
       
             
    }
    
    public static list<CampaignMember> getcList(){
        return clist;
    }
    
}