diff --git a/SDWebImage.podspec b/SDWebImage.podspec
index d478d200..20792483 100644
--- a/SDWebImage.podspec
+++ b/SDWebImage.podspec
@@ -62,6 +62,6 @@ Pod::Spec.new do |s|
       'USER_HEADER_SEARCH_PATHS' => '$(inherited) $(SRCROOT)/libwebp/src'
     }
     webp.dependency 'SDWebImage/Core'
-    webp.dependency 'libwebp'
+    webp.dependency 'libwebp', '~> 0.5'
   end
 end
diff --git a/SDWebImage/SDImageCache.m b/SDWebImage/SDImageCache.m
index 88b37c17..7eed11eb 100644
--- a/SDWebImage/SDImageCache.m
+++ b/SDWebImage/SDImageCache.m
@@ -11,31 +11,6 @@
 #import "NSImage+WebCache.h"
 #import "SDWebImageCodersManager.h"
 
-// See https://github.com/rs/SDWebImage/pull/1141 for discussion
-@interface AutoPurgeCache : NSCache
-@end
-
-@implementation AutoPurgeCache
-
-- (nonnull instancetype)init {
-    self = [super init];
-    if (self) {
-#if SD_UIKIT
-        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(removeAllObjects) name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
-#endif
-    }
-    return self;
-}
-
-- (void)dealloc {
-#if SD_UIKIT
-    [[NSNotificationCenter defaultCenter] removeObserver:self name:UIApplicationDidReceiveMemoryWarningNotification object:nil];
-#endif
-}
-
-@end
-
-
 FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 #if SD_MAC
     return image.size.height * image.size.width;
@@ -96,7 +71,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
         _config = [[SDImageCacheConfig alloc] init];
         
         // Init the memory cache
-        _memCache = [[AutoPurgeCache alloc] init];
+        _memCache = [[NSCache alloc] init];
         _memCache.name = fullNamespace;
 
         // Init the disk cache
diff --git a/SDWebImage/SDWebImageDownloader.h b/SDWebImage/SDWebImageDownloader.h
index ac7e5219..77a0ee8d 100644
--- a/SDWebImage/SDWebImageDownloader.h
+++ b/SDWebImage/SDWebImageDownloader.h
@@ -250,6 +250,7 @@ typedef SDHTTPHeadersDictionary * _Nullable (^SDWebImageDownloaderHeadersFilterB
  * Invalidates the managed session, optionally canceling pending operations.
  * @note If you use custom downloader instead of the shared downloader, you need call this method when you do not use it to avoid memory leak
  * @param cancelPendingOperations Whether or not to cancel pending operations.
+ * @note Calling this method on the shared downloader has no effect.
  */
 - (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations;
 
diff --git a/SDWebImage/SDWebImageDownloader.m b/SDWebImage/SDWebImageDownloader.m
index 472559ca..3adc937c 100644
--- a/SDWebImage/SDWebImageDownloader.m
+++ b/SDWebImage/SDWebImageDownloader.m
@@ -108,6 +108,9 @@
 }
 
 - (void)invalidateSessionAndCancel:(BOOL)cancelPendingOperations {
+    if (self == [SDWebImageDownloader sharedDownloader]) {
+        return;
+    }
     if (cancelPendingOperations) {
         [self.session invalidateAndCancel];
     } else {
diff --git a/SDWebImage/SDWebImageDownloaderOperation.m b/SDWebImage/SDWebImageDownloaderOperation.m
index 8fdab376..f8057041 100644
--- a/SDWebImage/SDWebImageDownloaderOperation.m
+++ b/SDWebImage/SDWebImageDownloaderOperation.m
@@ -419,7 +419,8 @@ didReceiveResponse:(NSURLResponse *)response
                             image = [[SDWebImageCodersManager sharedInstance] decompressedImageWithImage:image data:&imageData options:@{SDWebImageCoderScaleDownLargeImagesKey: @(shouldScaleDown)}];
                         }
                     }
-                    if (CGSizeEqualToSize(image.size, CGSizeZero)) {
+                    CGSize imageSize = image.size;
+                    if (imageSize.width == 0 || imageSize.height == 0) {
                         [self callCompletionBlocksWithError:[NSError errorWithDomain:SDWebImageErrorDomain code:0 userInfo:@{NSLocalizedDescriptionKey : @"Downloaded image has 0 pixels"}]];
                     } else {
                         [self callCompletionBlocksWithImage:image imageData:imageData error:nil finished:YES];
diff --git a/SDWebImage/SDWebImageGIFCoder.m b/SDWebImage/SDWebImageGIFCoder.m
index 69172104..e05d1c0b 100644
--- a/SDWebImage/SDWebImageGIFCoder.m
+++ b/SDWebImage/SDWebImageGIFCoder.m
@@ -72,7 +72,7 @@
             [frames addObject:frame];
         }
         
-        NSUInteger loopCount = 0;
+        NSUInteger loopCount = 1;
         NSDictionary *imageProperties = (__bridge_transfer NSDictionary *)CGImageSourceCopyProperties(source, nil);
         NSDictionary *gifProperties = [imageProperties valueForKey:(__bridge_transfer NSString *)kCGImagePropertyGIFDictionary];
         if (gifProperties) {