"Mutating method sent to immutable object" despite object being NSMutableDictionary - objective-c

Question

I'm using NSMutableDictionary and hit this error:
'NSInternalInconsistencyException', reason: '-[__NSCFDictionary removeObjectForKey:]: mutating method sent to immutable object'
Here's the code:
// Turn the JSON strings/data into objects
NSError *error;
NSMutableDictionary *invoiceDictFromReq = [[NSMutableDictionary alloc] init];
// invoiceDictFromReq = (NSMutableDictionary *)[NSJSONSerialization JSONObjectWithData:[request responseData] options:kNilOptions error:&error];
invoiceDictFromReq = [NSMutableDictionary dictionaryWithDictionary:[NSJSONSerialization JSONObjectWithData:[request responseData] options:kNilOptions error:&error]];
NSLog(#"invoiceDictFromReq count: %i, key: %#, value: %#", [invoiceDictFromReq count], [invoiceDictFromReq allKeys], [invoiceDictFromReq allValues]);
// Get values and keys from JSON response
self.invoiceDict = [invoiceDictFromReq objectForKey:#"invoice"];
NSNumber *invoiceAmount = [self.invoiceDict objectForKey:#"amount"];
NSNumber *invoiceId = [self.invoiceDict objectForKey:#"id"];
NSNumber *invoiceNumber = [self.invoiceDict objectForKey:#"number"];
NSNumber *checkoutStarted = [self.invoiceDict objectForKey:#"checkoutStarted"];
NSNumber *checkoutCompleted = [self.invoiceDict objectForKey:#"checkoutCompleted"];
NSLog(#"amount: %#, id: %#, number: %#, started: %#, completed: %#", invoiceAmount, invoiceId, invoiceNumber, checkoutStarted, checkoutCompleted);
All the console logs indicate that the data is fine. This is where things start to break down.
I pass the invoiceDict property to the next view controller:
// Pass the invoice to checkoutViewController
[checkoutViewController setInvoiceDict:self.invoiceDict];
In CheckoutViewController.m:
// Change invoice checkoutCompleted to true
// [self.invoiceDict removeObjectForKey:#"checkoutCompleted"];
[self.invoiceDict setObject:[NSNumber numberWithBool:YES] forKey:#"checkoutCompleted"];
The error is at [self.invoiceDict setObject...]. I made sure that all the dictionaries I use are NSMutableDictionary. I left some of the commented-out lines in the code to show the things I've tried and I hit a brick wall. I suppose I can always create a new dictionary. Is that the preferred way to do it?

Answer 1

NSJSONSerialization returns immutable objects by default. Here is how to get mutable dictionary from the parser:
use option NSJSONReadingMutableContainers
or
use mutableCopy on the result

Answer 2

You are allocing a dictionary in invoiceDictFromReq and next you are creating another dictionary, you are creating a leak of memory there.
Delete the line
NSMutableDictionary *invoiceDictFromReq = [[NSMutableDictionary alloc] init];
But your problem is that you are creating a NSMutableDictionary but you are setting to self.invoiceDict a dictionary inside your mutableDictionary, that is not necessarily a mutableDictionary too.
Change the line
self.invoiceDict = [invoiceDictFromReq objectForKey:#"invoice"];
for
self.invoiceDict = [NSMutableDictionary dictionaryWithDictionary:[invoiceDictFromReq objectForKey:#"invoice"]];