I am trying to test an HttpCallout using mocks and I get the following error :
 

System.TypeException: Supplied type is not an interface
Stack Trace	Class.System.Test.setMock: line 57, column 1
Class.ProjectCalloutServiceTest.testBillingCalloutService: line 15, column 1

 

@IsTest
private class ProjectCalloutServiceTest {

    @IsTest
    private static void testBillingCalloutService() {
        
        Account a = new Account(Name = 'Acme');
        insert a;
        Opportunity o = new Opportunity(Name='Test', StageName='Submitted Project', AccountId=a.Id, Amount=1000, CloseDate=Date.Today());
        insert o;
        
        List<Id> oppList = new List<Id>();
        oppList.add(o.Id);
  
        Test.setMock(ProjectCalloutServiceMock.class, new ProjectCalloutServiceMock());
        Test.startTest();
            ProjectCalloutService.postOpportunityToPMS(oppList);
        Test.stopTest();
        // runs callout and check results
        o = [select StageName from Opportunity where id =: o.Id];
        System.assertEquals('Submitted Project', o.StageName);
  }

 

isTest
global class ProjectCalloutServiceMock implements HttpCalloutMock {
    global HttpResponse respond(HttpRequest req) {
        // Create a fake response
        HttpResponse res = new HttpResponse();
        res.setHeader('Content-Type', 'application/json');
        res.setBody('{"status":"success"}');
        res.setStatusCode(200);
        return res; 
    }
}

I can't find any documentation on this error and what is causing that, could you please advise ?

Thanks a lot.