NSDictionary: validationhash fails to validate (Secure-NSUserDefaults) - ios

Question

I am using this helper class: Secure-NSUserDefaults
I'm running into a problem where the validationhash differs from the stored hash for a NSDictionary when I modify the latter and try saving it again.
Here is my code:
NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
BOOL valid = NO;
NSDictionary * modDict = [defaults secureDictionaryForKey:aKey valid:&valid];
if (!valid) {
//handle
}
for (NSString * modKey in modDict) {
NSMutableArray * modArray = [[modDict objectForKey:modKey] mutableCopy];
NSString * newValue = #"newValue";
[modArray replaceObjectAtIndex:0 withObject:newValue];
NSMutableDictionary *newModDict = [modDict mutableCopy];
[newModDict setObject:modArray forKey:modKey];
[defaults setSecureObject:newModDict forKey:aKey];
[defaults synchronize];
}
When I run this code once, kill the app, and start it again, all works as it should, and the validationhash matches. It always matches correctly on the first run, in fact. In other words: my code saves out the secure dictionary and the hash matches on the subsequent app run just fine (once).
BUT - if I run the same code a second time without killing the app between runs, the validationhash is different, and the &valid check fails.
I was able to trace down the problem to this line:
[newModDict setObject:modArray forKey:modKey];
If I comment it out, it works fine all the time, but then of course, the modification I am trying to make isn't made at all, I simply copy the Dictionary "as is".
I'm struggling to figure out what I am doing wrong or how to modify and re-save the NSDictionary so that the hash matches.
Thanks in advance for any help!

Answer 1

LOL! Funny how sometimes writing down a question in a concise way triggers new ideas. It just occurred to me what I had to do, and just now solved it like this:
[newModDict setObject:[modArray copy] forKey:modKey];
i.e. I had to make the edited NSArray non-mutable first.
Maybe someone will benefit from the answer. I'm too embarrassed to mention how long I worked on this last night.