NSCache getting released randomly

so I had a category of NSCache:

@interface NSCache (SharedCache)
 +(NSCache*)sharedCache;
 @end
 
 #import "NSCache+SharedCache.h"
 
 @implementation NSCache (SharedCache)
 +(NSCache*)sharedCache;
 {
     static NSCache* sharedCache = nil;
     if (sharedCache == nil) {
         sharedCache = [[self alloc] init];
         [sharedCache setCountLimit:0];
     }
     return sharedCache;
 }
 
 @end
 

However ocasionnaly this gets deallocated and therefore my app crashes. If I retain sharedCache this problem is solved. What is the best way to have a shared NSCache to void crashes that it gets deallocated randomly?

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