Hash value of NSDictionary

I ran into an issue, where I got the same hash value for different dictionaries. Maybe I'm doing something obvious wrong, but I thought, objects with different content (a.k.a. not equal objects) should have different hash values.

NSDictionary *dictA = @{ @"foo" : @YES };
NSDictionary *dictB = @{ @"foo" : @NO };

BOOL equal = [dictA hash] == [dictB hash];

NSAssert(!equal, @"Assuming, that different dictionaries have different hash values.");

Any thoughts?