diff --git a/SDWebImage/SDWebImageDecoder.m b/SDWebImage/SDWebImageDecoder.m
index 80019ff3..2bb5472f 100644
--- a/SDWebImage/SDWebImageDecoder.m
+++ b/SDWebImage/SDWebImageDecoder.m
@@ -19,14 +19,16 @@
     // when there are memory warning.
     // on iOS7, do not forget to call
     // [[SDImageCache sharedImageCache] clearMemory];
-
+    
     if (image == nil) { // Prevent "CGBitmapContextCreateImage: invalid context 0x0" error
         return nil;
     }
-
+    
     @autoreleasepool{
         // do not decode animated images
-        if (image.images) { return image; }
+        if (image.images != nil) {
+            return image;
+        }
         
         CGImageRef imageRef = image.CGImage;
         
@@ -35,43 +37,55 @@
                          alpha == kCGImageAlphaLast ||
                          alpha == kCGImageAlphaPremultipliedFirst ||
                          alpha == kCGImageAlphaPremultipliedLast);
-        
-        if (anyAlpha) { return image; }
+        if (anyAlpha) {
+            return image;
+        }
         
         // current
         CGColorSpaceModel imageColorSpaceModel = CGColorSpaceGetModel(CGImageGetColorSpace(imageRef));
         CGColorSpaceRef colorspaceRef = CGImageGetColorSpace(imageRef);
         
-        bool unsupportedColorSpace = (imageColorSpaceModel == 0 || imageColorSpaceModel == -1 || imageColorSpaceModel == kCGColorSpaceModelCMYK || imageColorSpaceModel == kCGColorSpaceModelIndexed);
-        if (unsupportedColorSpace)
+        BOOL unsupportedColorSpace = (imageColorSpaceModel == kCGColorSpaceModelUnknown ||
+                                      imageColorSpaceModel == kCGColorSpaceModelMonochrome ||
+                                      imageColorSpaceModel == kCGColorSpaceModelCMYK ||
+                                      imageColorSpaceModel == kCGColorSpaceModelIndexed);
+        if (unsupportedColorSpace) {
             colorspaceRef = CGColorSpaceCreateDeviceRGB();
+        }
         
         size_t width = CGImageGetWidth(imageRef);
         size_t height = CGImageGetHeight(imageRef);
         NSUInteger bytesPerPixel = 4;
         NSUInteger bytesPerRow = bytesPerPixel * width;
         NSUInteger bitsPerComponent = 8;
-        
+
+
+        // kCGImageAlphaNone is not supported in CGBitmapContextCreate.
+        // Since the original image here has no alpha info, use kCGImageAlphaNoneSkipLast
+        // to create bitmap graphics contexts without alpha info.
         CGContextRef context = CGBitmapContextCreate(NULL,
                                                      width,
                                                      height,
                                                      bitsPerComponent,
                                                      bytesPerRow,
                                                      colorspaceRef,
-                                                     kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedFirst);
+                                                     kCGBitmapByteOrderDefault|kCGImageAlphaNoneSkipLast);
         
-        // Draw the image into the context and retrieve the new image, which will now have an alpha layer
+        // Draw the image into the context and retrieve the new bitmap image without alpha
         CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
-        CGImageRef imageRefWithAlpha = CGBitmapContextCreateImage(context);
-        UIImage *imageWithAlpha = [UIImage imageWithCGImage:imageRefWithAlpha scale:image.scale orientation:image.imageOrientation];
+        CGImageRef imageRefWithoutAlpha = CGBitmapContextCreateImage(context);
+        UIImage *imageWithoutAlpha = [UIImage imageWithCGImage:imageRefWithoutAlpha
+                                                         scale:image.scale
+                                                   orientation:image.imageOrientation];
         
-        if (unsupportedColorSpace)
+        if (unsupportedColorSpace) {
             CGColorSpaceRelease(colorspaceRef);
+        }
         
         CGContextRelease(context);
-        CGImageRelease(imageRefWithAlpha);
+        CGImageRelease(imageRefWithoutAlpha);
         
-        return imageWithAlpha;
+        return imageWithoutAlpha;
     }
 }