diff --git a/SDWebImage/SDWebImageManager.h b/SDWebImage/SDWebImageManager.h
index 736fbba8..da8abcec 100644
--- a/SDWebImage/SDWebImageManager.h
+++ b/SDWebImage/SDWebImageManager.h
@@ -142,7 +142,9 @@ SDWebImageManager *manager = [SDWebImageManager sharedManager];
  * @param progressBlock A block called while image is downloading
  * @param completedBlock A block called when operation has been completed.
  *
- *   This block as no return value and takes the requested UIImage as first parameter.
+ *   This parameter is required.
+ * 
+ *   This block has no return value and takes the requested UIImage as first parameter.
  *   In case of error the image parameter is nil and the second parameter may contain an NSError.
  *
  *   The third parameter is an `SDImageCacheType` enum indicating if the image was retrived from the local cache
diff --git a/SDWebImage/SDWebImageManager.m b/SDWebImage/SDWebImageManager.m
index 7e241377..8940d656 100644
--- a/SDWebImage/SDWebImageManager.m
+++ b/SDWebImage/SDWebImageManager.m
@@ -68,6 +68,9 @@
 
 - (id<SDWebImageOperation>)downloadWithURL:(NSURL *)url options:(SDWebImageOptions)options progress:(SDWebImageDownloaderProgressBlock)progressBlock completed:(SDWebImageCompletedWithFinishedBlock)completedBlock
 {    
+    // Invoking this method without a completedBlock is pointless
+    NSParameterAssert(completedBlock);
+    
     // Very common mistake is to send the URL using NSString object instead of NSURL. For some strange reason, XCode won't
     // throw any warning for this type mismatch. Here we failsafe this error by allowing URLs to be passed as NSString.
     if ([url isKindOfClass:NSString.class])
@@ -90,16 +93,13 @@
         isFailedUrl = [self.failedURLs containsObject:url];
     }
 
-    if (!url || !completedBlock || (!(options & SDWebImageRetryFailed) && isFailedUrl))
+    if (!url || (!(options & SDWebImageRetryFailed) && isFailedUrl))
     {
-        if (completedBlock)
+        dispatch_main_sync_safe(^
         {
-            dispatch_main_sync_safe(^
-            {
                 NSError *error = [NSError errorWithDomain:NSURLErrorDomain code:NSURLErrorFileDoesNotExist userInfo:nil];
                 completedBlock(nil, error, SDImageCacheTypeNone, YES);
-            });
-        }
+        });
         return operation;
     }