If there is a chance that an NSArray is empty, is it better to check it and set equal to nil if it's empty when it is assigned or to rather do the check when it is used?

e.g.

NSArray *myArray;

if ([anotherArray count] > 0)     <-- Check when assigned
  myArray = [anotherArray copy];
else
  myArray = nil;

something = [myArray objectAtIndex:x];

or

NSArray *myArray;

myArray = [anotherArray copy];

if ([myArray count] > 0)          <-- Check when used
  something = [myArray objectAtIndex:x];

Which is better?