Unrecognized selector error when trying to access a specific object property

I have a little problem, probably easy for you. Im using Core Data. I have an entity: Session, which have three attributes: access_token, user_id and secret.

Session.h:

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@interface Session : NSManagedObject

@property (nonatomic) NSString * access_token;
@property (nonatomic) NSNumber * user_id;
@property (nonatomic) NSString * secret;

@end

Session.m:

#import "Session.h"

@implementation Session

@dynamic access_token;
@dynamic user_id;
@dynamic secret;

@end 

There is a code, where im using this entity:

AFJSONRequestOperation *operation = [AFJSONRequestOperation JSONRequestOperationWithRequest:request success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
    Session *newSession = [NSEntityDescription insertNewObjectForEntityForName:@"Session" inManagedObjectContext:managedObjectContext];
    if (newSession != nil) {
        NSLog(@"access_token: %@", [JSON objectForKey:@"access_token"]);
        NSLog(@"user_id: %@", [JSON objectForKey:@"user_id"]);
        NSLog(@"secret: %@", [JSON objectForKey:@"secret"]);
        newSession.access_token = [JSON objectForKey:@"access_token"];
        newSession.user_id = [JSON objectForKey:@"user_id"];
        newSession.secret = [JSON objectForKey:@"secret"];
    }
    NSError *savingError = nil;
    if ([managedObjectContext save:&savingError] == YES) {
        NSLog(@"Session saved");
    } else {
        NSLog(@"Session not saved");
    }
} failure:nil];

This code make an exception:

newSession.secret = [JSON objectForKey:@"secret"];

MyProject[7678:bc03] -[Session setSecret:]: unrecognized selector sent to instance 0x4e572d0

This is NSLog:

2012-03-25 15:17:33.987 MyProject[7730:bc03] access_token: <some_access_token>
2012-03-25 15:17:33.988 MyPriject[7730:bc03] user_id: <some_user_id>
2012-03-25 15:17:33.989 MyProject[7730:bc03] secret: <some_secret>

This is stack trace:

*** Call stack at first throw:
(
0   CoreFoundation                      0x010625a9 __exceptionPreprocess + 185
1   libobjc.A.dylib                     0x013e6313 objc_exception_throw + 44
2   CoreFoundation                      0x010640bb -[NSObject(NSObject) doesNotRecognizeSelector:] + 187
3   CoreFoundation                      0x00fd3966 ___forwarding___ + 966
4   CoreFoundation                      0x00fd3522 _CF_forwarding_prep_0 + 50
5   MyProject                           0x00004c94 __42-[APIWrapper authWithLogin:andPassword:]_block_invoke_0 + 836
6   MyProject                           0x0000d452 __74+[AFJSONRequestOperation JSONRequestOperationWithRequest:success:failure:]_block_invoke_0 + 146
7   MyProject                           0x0000e229 __block_global_3 + 41
8   libdispatch_sim.dylib               0x014de289 _dispatch_call_block_and_release + 16
9   libdispatch_sim.dylib               0x014e1833 _dispatch_main_queue_callback_4CF + 312
10  CoreFoundation                      0x00fa1589 __CFRunLoopRun + 2521
11  CoreFoundation                      0x00fa0840 CFRunLoopRunSpecific + 208
12  CoreFoundation                      0x00fa0761 CFRunLoopRunInMode + 97
13  GraphicsServices                    0x0153a1c4 GSEventRunModal + 217
14  GraphicsServices                    0x0153a289 GSEventRun + 115
15  UIKit                               0x000ebc93 UIApplicationMain + 1160
16  MyProject                           0x0001b305 main + 181
17  MyProject                           0x000020d5 start + 53
)

Im really dont understand what's problem here.

In your .h file define your properties like this.

@property (nonatomic,retain) NSString * access_token;
@property (nonatomic,retain) NSNumber * user_id;
@property (nonatomic,retain) NSString * secret;

And then in your .m file

@synthesize access_token;
@synthesize user_id;
@synthesize secret;

Basically this manages your getters and setters.