iOS nsmutalblearray addobject程序崩溃

请问NSMutableArray在alloc init之后再调用addobject还是会崩溃的可能原因有哪些？
出现错误：[__NSArrayI addObject:]: unrecognized selector sent to instance 0xb848630'

ChatMessage *newMsg = [[ChatMessage alloc] init];
newMsg.owner = MSG_OWNER_SELF;
newMsg.type = MSG_TYPE_SOUND;
newMsg.timeStamp = self.dateToRecordStr;
newMsg.msg = [NSString stringWithFormat:@"%@%@.aac", self.toChatUsr.UID, newMsg.timeStamp];
[self.toChatUsr.msgs addObject:newMsg];

这是代码
最后一句会崩掉。
toChatUsr是 @property(weak, nonatomic)，赋值是 = 一个@property(strong, nonatomic)

chatMsssage的初始化代码

@implementation ChatMessage

(id)init{
self = [super init];
if (self) {
self.owner = [[NSString alloc] init];
self.type = [[NSString alloc] init];
self.timeStamp = [[NSString alloc] init];
self.msg = [[NSString alloc] init];
}
return self;
}