response data returning null when url works

Question

I'm trying to get some data sent from my app to a web server.

What the app is supposed to do is display a list of members, show their details when selected and give the ability to edit their details. so far everything except for editing is working.

The error message i'm getting is

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'data parameter is nil'

The app works fine up until

NSDictionary* json = [NSJSONSerialization JSONObjectWithData:responseData options:kNilOptions error:&error];

It appears that responseData is null. I can't understand it since the url that is generated works fine when entered into the browser.

This is the whole code for the updating details

- (IBAction)submitEdits:(id)sender {
    smdbs_AppData *appData = [smdbs_AppData sharedInstance];
    NSString *loggedInID=appData.loggedInID;

    NSString *member = [self.memberDetailModel objectAtIndex:3];

    NSURL *urlLogin = [NSURL URLWithString:[NSString stringWithFormat:@"http://xxx/API.php?task=setMemberDetails&id=%@&mem_id=%@&section=%@&firstName=%@&lastName=%@&membershipNo=%@&DOB=%@&gender=%@&school=%@&religion=%@&ethnicity=%@&photoPerm=%@&swimPerm=%@&giftaid=%@&giftaidperson=%@&address1=%@&address2=%@&town=%@&county=%@&postCode=%@&homeTelNo=%@&mother=%@&motherMobile=%@&father=%@&fatherMobile=%@&other=%@&relationship=%@&otherTelNo=%@&email1=%@&email2=%@&doctor=%@&surgery=%@&docTelNo=%@&allergiesMed=%@&specialNeeds=%@&pack=%@&six=%@&joinedScouting=%@&joinedBeavers=%@&joinedCubs=%@&joinedScouts=%@&joinedYLs=%@&joinedHelpers=%@&joinedLeaders=%@&crbDate=%@&crbNo=%@", loggedInID, member, self.section.text, self.firstName.text, self.lastName.text, self.membershipNo.text, self.DOB.text, self.gender.text, self.school.text, self.religion.text, self.ethnicity.text, self.photoPerm.text, self.swimPerm.text, self.giftaid.text, self.giftaidperson.text, self.address1.text, self.address2.text, self.town.text, self.county.text, self.postCode.text, self.homeTelNo.text, self.mother.text, self.motherMobile.text, self.father.text, self.fatherMobile.text, self.other.text, self.relationship.text, self.otherTelNo.text, self.email1.text, self.email2.text, self.doctor.text, self.surgery.text, self.docTelNo.text, self.allergiesMed.text, self.specialNeeds.text, self.pack.text, self.six.text, self.joinedScouting.text, self.joinedBeavers.text, self.joinedCubs.text, self.joinedScouts.text, self.joinedYLs.text, self.joinedHelpers.text, self.joinedLeaders.text, self.crbDate.text, self.crbNo.text]];

    dispatch_async(kBgQueue, ^{
        NSData* editData = [NSData dataWithContentsOfURL:urlLogin];
        [self performSelectorOnMainThread:@selector(EditData:)
                               withObject:editData waitUntilDone:YES];
    });
}

- (void)EditData:(NSData *)responseData
{
    NSError* error;
    NSLog(@"%@",responseData);
    NSDictionary* json = [NSJSONSerialization JSONObjectWithData:responseData options:kNilOptions error:&error];
    NSString* success =  [json objectForKey:@"success"];

    if ([success isEqualToString:@"1"]) {
        [self performSegueWithIdentifier:@"showMemberDetails" sender:nil];
    }
}