Core MemoryCache Get method is null

Hi,

Why is the MemoryCache's Get method is always null?

using Microsoft.Extensions.Caching.Memory; //PM> CacheManager.Microsoft.Extensions.Caching.Memory -Version 1.1.0

    public class IndexController
    {
        IMemoryCache _memoryCache;

        public IndexController(IMemoryCache memoryCache)
        {
            _memoryCache = memoryCache;
        }

        public IActionResult Index(string key)
        {
		object isCached = _memoryCache.Get(key); //<<--always null on every request 

		if (isCached != null){
			//cache item

		object cacheEntry;

		string data = "string-data-here";

            // Look for cache key.
            if (!_memoryCache.TryGetValue(key, out cacheEntry))
            {
                // Key not in cache, so get data.
                cacheEntry = data;

                // Set cache options.
                var cacheEntryOptions = new MemoryCacheEntryOptions()
                    // Keep in cache for this time, reset time if accessed.
                    .SetSlidingExpiration(TimeSpan.FromMinutes(60));

                // Save data in cache.
                _memoryCache.Set(key, cacheEntry, cacheEntryOptions);
            }
		}
	}

Thanks,