Why NSLog of [someObject class] may generate crashes ?

Why NSLog(@"%@",[someObject class]) may crash an application ?
If someObject is not an instance of a class descending from NSObject, the root class may not implement the required methods to support being used in string format operation:
+ (id)description;
and a private method:
+(NSString*)_copyDescription;
Example: this is the case with NSProxy class.