If some parameter is missing in json or dictionary. For example-

json = (
 {
            "created_time" = "2013-11-18T14:40:37+0000";
            from =                 {
                id = 15400100;
                name = "Alexandra";
            };
            id = "430737890381371_26";
            message = "adding Emily...can we dress up as politically incorrect native americans and pilgrims for this? ";
        },
                    {
            "created_time" = "2013-11-18T18:39:49+0000";
            from =                 {
                id = 793231520;
                name = "Danielle";
            };
            id = "430737890381371_27";
        },
                    {
            "created_time" = "2013-11-18T20:47:47+0000";
            from =                 {
                id = 100000375798578;
                name = "Kutney";
            };
            id = "430737890381371_29";
            message = "Sounds super. Wish I was in town.";
        },

)

In the above example the message param is missing. To resolve or identify this, we should put a condition. Please see the solution below..

Solution is :-

NSString *text = [NSString stringWithFormat:@"%@",[[json valueForKey:@"message"] objectAtIndex:i]];
                if ([text isEqualToString:@"<null>"] || text.length<=0) {
                   // Do your code here.....
                }