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[NSInvalidArgumentException] NSMutableDictionaryのsetObjectではnilをsetすると落ちる。NSNullを使いなさい

これに、はまって大量の、Exeptionを出してしまった。

- (void)setObject:(id)anObject forKey:(id < NSCopying >)aKey
Parameters
anObject
The value for aKey. A strong reference to the object is maintained by the dictionary. 
Raises an NSInvalidArgumentException if anObject is nil. If you need to represent a nil value in the dictionary, use NSNull.
aKey
The key for value. The key is copied (using copyWithZone:; keys must conform to the NSCopying protocol). 
Raises an NSInvalidArgumentException if aKey is nil.
 If aKey already exists in the dictionary anObject takes its place.

Availability
Available in iOS 2.0 and later.