objective c - Calling NSMutableArray:addObject is resulting in an unrecognized selector exception -

i'm getting unrecognized selector exception attempting call addobject on array. array nsmutablearray, not nsarray. don't understand why happening.

i have following classes:

@interface : nsobject @property (nonatomic, copy)     nsmutablearray* children; - (id) init; - (void) addchild: (a*) child; @end  @interface b : - (id) init; @end  @implementation - (id) init  {     self = [super init];    return self; }   - (void) addchild: (a*) child {     [self.children addobject:child]; } @end   @implementation b -(id) init  {     self = [super init];     if (self != nil)     {            self.children = [[nsmutablearray alloc] init];     }     return (self); } 

then if create object of type b , call [b addchild: anobject] i'm getting unrecognized selector , don't understand why.

the output is:

2013-04-08 14:31:08.046 otest[13381:7e03] -[__nsarrayi addobject:]: unrecognized selector sent instance 0x17759c0 

what problem?

tia

(please focus on actual code , problem, not side discussions why i'm not allocating array in a's init, unless relevant. want understand why code stands does't work. thanks).

by declaring array storage qualifier copy instead of strong, mutable array copied on assignment , turned immutable nsarray.