Objective c add string object to an array

I've looked everywhere for this, online, on stack overflow and cannot still work out what I'm doing wrong.

I'm trying to add an element to an existing NSMutableArray. But it crashes on line 4:

-[__NSArrayI addObject:]: unrecognized selector sent to instance 0x897b320

The code:

NSMutableArray *mystr = [[NSMutableArray alloc] init];

mystr = [NSArray arrayWithObjects:@"hello",@"world",@"etc",nil];

NSString *obj = @"hiagain";

[mystr addObject:obj];

What am I doing wrong? This is driving me crazy!!!