I have code that does:

NSXMLParser *xmlDoc = [[NSXMLParser alloc] initWithContentsOfURL:furl];
[xmlDoc parse];

Running on iOS 6, this works. Running on iOS 5, the parse call destroys the stack with "dyld: Symbol not found: _objc_setProperty_nonatomic".

What does this mean and how do I fix it?

My settings are "Base SDK: Latest iOS (6.1)", "iOS Deployment Target: iOS 4.3". Some googling suggested changing the latter to 6.1, but that changes nothing. What is the "deployment target", anyway? If I set it to 6.1, doesn't that mean the app will not run on anything earlier than 6.1?

As far as I know, this code should work all the way back to iOS 3.0, but Apple won't even let you target that any more. I'd rather not ship binaries that artificially require later versions of the OS than is strictly necessary.

Build environment is Xcode 4.6.2 on OSX 10.8.3.