this this class is not key value

2015-10-28 16:31:40.478 attendance[10644:128666] *** Terminating app due to uncaught exception 'NSUnknownKeyException', reason: '[<__NSDictionaryI 0x7fe0e047d340> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key pstype.'
错误如上，意思是该字典没有这个键，异常抛出是在这，
[socket.writeData setValue:@(20152)
forKey:@"pstype"];
socket.writeData是可变的字典，添加新的键值时需要进行内存分配，最终如下解决
socket.writeData=[NSMutableDictionary dictionaryWithCapacity:10];

[socket.writeData setValue:@(20152)
forKey:@"pstype"];