iphone - valueforkey when object property is readonly and Calculated(derived)

Question

I am having trouble calling valueforkey method on a property that is calculated. For example:

@interface Value : NSObject
@property(nonatomic,getter = toString,readonly)NSString *stringVal;

-(NSString*) toString;
@end

When I call [instanceOfValueClass valueForKey:@"stringVal"] , It says it is not key-value compliant. Exact Message is :

[ valueForUndefinedKey:]: this class is not key value coding-compliant for the key stringVal.

Does key value coding works on derived properties?

Thanks