iOS: Why can't I set nil to NSDictionary value?

It wants an actual object... use NSNull

[NSNull null];

You can set a nil value using setValue:forKey but it removes the key.

If you want to be able to set a key to nil you could use setValue:forKey: which will remove the key if you set it to nil (quote from documentation below). Note the Value instead of Object.

setValue:forKey:

Adds a given key-value pair to the dictionary.

...

Discussion

This method adds value and key to the dictionary using setObject:forKey:, unless value is nil in which case the method instead attempts to remove key using removeObjectForKey:.

When you later try and get the object using objectForKey: for the key that you removed by setting it to nil you will get nil back (quote from documentation below).

Return value:

The value associated with aKey, or nil if no value is associated with aKey.

Note: The key will not actually be present in the dictionary so it won't be obtained using allKeys; or be enumerated over.

You can set nil object in this way:

NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];

dictionary[@“key”] = nil;

Have you noticed it?

NSMutableDictionary *dictionary = [NSMutableDictionary dictionary];

/* this statement is safe to execute    */

dictionary[@“key”] = nil;

/* but this statement will crash application    */

[dictionary setObject:nil forKey:@"key"];