diff --git a/SDWebImage.xcodeproj/project.pbxproj b/SDWebImage.xcodeproj/project.pbxproj
index bbb554c1..3e2fefd6 100644
--- a/SDWebImage.xcodeproj/project.pbxproj
+++ b/SDWebImage.xcodeproj/project.pbxproj
@@ -112,6 +112,8 @@
 		327054D6206CD8B3006EA328 /* SDImageAPNGCoder.h in Headers */ = {isa = PBXBuildFile; fileRef = 327054D2206CD8B3006EA328 /* SDImageAPNGCoder.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		327054DA206CD8B3006EA328 /* SDImageAPNGCoder.m in Sources */ = {isa = PBXBuildFile; fileRef = 327054D3206CD8B3006EA328 /* SDImageAPNGCoder.m */; };
 		327054DC206CD8B3006EA328 /* SDImageAPNGCoder.m in Sources */ = {isa = PBXBuildFile; fileRef = 327054D3206CD8B3006EA328 /* SDImageAPNGCoder.m */; };
+		327F2E83245AE1650075F846 /* SDWebImageOperation.m in Sources */ = {isa = PBXBuildFile; fileRef = 327F2E82245AE1650075F846 /* SDWebImageOperation.m */; };
+		327F2E84245AE1650075F846 /* SDWebImageOperation.m in Sources */ = {isa = PBXBuildFile; fileRef = 327F2E82245AE1650075F846 /* SDWebImageOperation.m */; };
 		328BB69E2081FED200760D6C /* SDWebImageCacheKeyFilter.h in Headers */ = {isa = PBXBuildFile; fileRef = 328BB69A2081FED200760D6C /* SDWebImageCacheKeyFilter.h */; settings = {ATTRIBUTES = (Public, ); }; };
 		328BB6A22081FED200760D6C /* SDWebImageCacheKeyFilter.m in Sources */ = {isa = PBXBuildFile; fileRef = 328BB69B2081FED200760D6C /* SDWebImageCacheKeyFilter.m */; };
 		328BB6A42081FED200760D6C /* SDWebImageCacheKeyFilter.m in Sources */ = {isa = PBXBuildFile; fileRef = 328BB69B2081FED200760D6C /* SDWebImageCacheKeyFilter.m */; };
@@ -437,6 +439,7 @@
 		326E2F32236F1D58006F847F /* SDDeviceHelper.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; path = SDDeviceHelper.m; sourceTree = "<group>"; };
 		327054D2206CD8B3006EA328 /* SDImageAPNGCoder.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = SDImageAPNGCoder.h; path = Core/SDImageAPNGCoder.h; sourceTree = "<group>"; };
 		327054D3206CD8B3006EA328 /* SDImageAPNGCoder.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; name = SDImageAPNGCoder.m; path = Core/SDImageAPNGCoder.m; sourceTree = "<group>"; };
+		327F2E82245AE1650075F846 /* SDWebImageOperation.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; name = SDWebImageOperation.m; path = Core/SDWebImageOperation.m; sourceTree = "<group>"; };
 		328BB69A2081FED200760D6C /* SDWebImageCacheKeyFilter.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; name = SDWebImageCacheKeyFilter.h; path = Core/SDWebImageCacheKeyFilter.h; sourceTree = "<group>"; };
 		328BB69B2081FED200760D6C /* SDWebImageCacheKeyFilter.m */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.objc; name = SDWebImageCacheKeyFilter.m; path = Core/SDWebImageCacheKeyFilter.m; sourceTree = "<group>"; };
 		328BB6A82081FEE500760D6C /* SDWebImageCacheSerializer.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; name = SDWebImageCacheSerializer.h; path = Core/SDWebImageCacheSerializer.h; sourceTree = "<group>"; };
@@ -846,6 +849,7 @@
 				320CAE132086F50500CFFC80 /* SDWebImageError.h */,
 				320CAE142086F50500CFFC80 /* SDWebImageError.m */,
 				530E49E71646388E002868E7 /* SDWebImageOperation.h */,
+				327F2E82245AE1650075F846 /* SDWebImageOperation.m */,
 				324DF4B2200A14DC008A84CC /* SDWebImageDefine.h */,
 				324DF4B3200A14DC008A84CC /* SDWebImageDefine.m */,
 				325312C6200F09910046BF1E /* SDWebImageTransition.h */,
@@ -1196,6 +1200,7 @@
 				4A2CAE201AB4BB6C00B6BC39 /* SDImageCache.m in Sources */,
 				4369C2801D9807EC007E863A /* UIView+WebCache.m in Sources */,
 				329A18611FFF5DFD008C9A2F /* UIImage+Metadata.m in Sources */,
+				327F2E84245AE1650075F846 /* SDWebImageOperation.m in Sources */,
 				328BB6B22081FEE500760D6C /* SDWebImageCacheSerializer.m in Sources */,
 				325C4611223394D8004CAE11 /* SDImageCachesManagerOperation.m in Sources */,
 			);
@@ -1270,6 +1275,7 @@
 				ABBE71A818C43B4D00B75E91 /* UIImageView+HighlightedWebCache.m in Sources */,
 				4369C27E1D9807EC007E863A /* UIView+WebCache.m in Sources */,
 				329A185F1FFF5DFD008C9A2F /* UIImage+Metadata.m in Sources */,
+				327F2E83245AE1650075F846 /* SDWebImageOperation.m in Sources */,
 				328BB6B02081FEE500760D6C /* SDWebImageCacheSerializer.m in Sources */,
 				325C4610223394D8004CAE11 /* SDImageCachesManagerOperation.m in Sources */,
 			);
diff --git a/SDWebImage/Core/SDWebImageOperation.m b/SDWebImage/Core/SDWebImageOperation.m
new file mode 100644
index 00000000..0d6e880d
--- /dev/null
+++ b/SDWebImage/Core/SDWebImageOperation.m
@@ -0,0 +1,14 @@
+/*
+ * This file is part of the SDWebImage package.
+ * (c) Olivier Poitrey <rs@dailymotion.com>
+ *
+ * For the full copyright and license information, please view the LICENSE
+ * file that was distributed with this source code.
+ */
+
+#import "SDWebImageOperation.h"
+
+/// NSOperation conform to `SDWebImageOperation`.
+@implementation NSOperation (SDWebImageOperation)
+
+@end
diff --git a/SDWebImage/Core/SDWebImagePrefetcher.m b/SDWebImage/Core/SDWebImagePrefetcher.m
index efcb5c73..30b65ace 100644
--- a/SDWebImage/Core/SDWebImagePrefetcher.m
+++ b/SDWebImage/Core/SDWebImagePrefetcher.m
@@ -266,16 +266,18 @@
     @synchronized (self) {
         [self.prefetchOperations compact];
         for (id operation in self.prefetchOperations) {
-            if ([operation conformsToProtocol:@protocol(SDWebImageOperation)]) {
-                [operation cancel];
+            id<SDWebImageOperation> strongOperation = operation;
+            if (strongOperation) {
+                [strongOperation cancel];
             }
         }
         self.prefetchOperations.count = 0;
         
         [self.loadOperations compact];
         for (id operation in self.loadOperations) {
-            if ([operation conformsToProtocol:@protocol(SDWebImageOperation)]) {
-                [operation cancel];
+            id<SDWebImageOperation> strongOperation = operation;
+            if (strongOperation) {
+                [strongOperation cancel];
             }
         }
         self.loadOperations.count = 0;
diff --git a/SDWebImage/Private/SDAsyncBlockOperation.m b/SDWebImage/Private/SDAsyncBlockOperation.m
index 8862ef8e..4aefb4f5 100644
--- a/SDWebImage/Private/SDAsyncBlockOperation.m
+++ b/SDWebImage/Private/SDAsyncBlockOperation.m
@@ -52,7 +52,9 @@
 
 - (void)cancel {
     [super cancel];
-    [self complete];
+    if (self.isExecuting) {
+        [self complete];
+    }
 }
 
 - (void)complete {