AddThis

Undefined symbols for architecture armv6...

Trying to add the "AddThis" library for the first time to an existing XCode project. When building I get link errors:

Undefined symbols for architecture armv6:
"_OBJC_CLASS_$_NSAttributeDescription", referenced from:
objc-class-ref in libAddThis.a(ATCoreController.o)
"_OBJC_CLASS_$_NSRelationshipDescription", referenced from:
objc-class-ref in libAddThis.a(ATCoreController.o)
"_OBJC_CLASS_$_NSFetchRequest", referenced from:
objc-class-ref in libAddThis.a(ATCoreController.o)
objc-class-ref in libAddThis.a(ATFavouriteListViewController.o)
objc-class-ref in libAddThis.a(ATAllServicesViewController.o)
objc-class-ref in libAddThis.a(ATCacheUpdater.o)
"_NSManagedObjectContextDidSaveNotification", referenced from:
-[ATCacheUpdater startCacheUpdate] in libAddThis.a(ATCacheUpdater.o)
"_OBJC_CLASS_$_NSManagedObjectContext", referenced from:
objc-class-ref in libAddThis.a(ATCoreController.o)
objc-class-ref in libAddThis.a(ATCacheUpdater.o)
"_NSSQLiteStoreType", referenced from:
-[ATCoreController persistentStoreCoordinator] in libAddThis.a(ATCoreController.o)
"_OBJC_CLASS_$_NSManagedObjectModel", referenced from:
objc-class-ref in libAddThis.a(ATCoreController.o)
"_OBJC_CLASS_$_NSEntityDescription", referenced from:
objc-class-ref in libAddThis.a(ATCoreController.o)
objc-class-ref in libAddThis.a(ATFavouriteListViewController.o)
objc-class-ref in libAddThis.a(ATAllServicesViewController.o)
objc-class-ref in libAddThis.a(ATCacheUpdater.o)
"_OBJC_CLASS_$_NSPersistentStoreCoordinator", referenced from:
objc-class-ref in libAddThis.a(ATCoreController.o)
ld: symbol(s) not found for architecture armv6
collect2: ld returned 1 exit status