Regarding over-releasing. Say I have a instance variable defined in Test.h

NSString *mystring;

In my implementation Test.m I do not initialize the variable mystring anywhere. But I release it in dealloc:

-(void)dealloc {
    [mystring release];
}

Is this now over-released? I've been doing the following in dealloc to avoid any issues, however, is this really necessary?

-(void)dealloc {
     if (mystring) [mystring release];
}

It seems that [nil release] shouldn't do anything, can someone verify this with class members?