CoreData: error: Failed to call designated initializer on NSManagedObject class

I have a little damn problem with CoreData. I want to insert a new Object, so I first have to create one. This is done by that code:

Challenges *newChallenge = [[Challenges alloc] init];
[newChallenge setName:@"TestChallenge"];
[newChallenge setRounds:[[NSNumber alloc] initWithInt:12]];
[newChallenge setShots:[[NSNumber alloc] initWithInt:5]];
[newChallenge setDate:[NSDate date]];

But however after the alloc init I get this error:

CoreData: error: Failed to call designated initializer on NSManagedObject class 'Challenges'

What the hack is going wrong?