Objective-c – iPhone development: pointer being freed was not allocated

i got this message from the debugger:

Pixture(1257,0xa0610500) malloc: *** error for object 0x21a8000: pointer being freed was not allocated
*** set a breakpoint in malloc_error_break to debug

so i did a bit of tracing and got:

(gdb) shell malloc_history 1257 0x21a8000

ALLOC 0x2196a00-0x21a89ff [size=73728]: thread_a0610500 |start | main | UIApplicationMain | GSEventRun | GSEventRunModal | CFRunLoopRunInMode | CFRunLoopRunSpecific | __CFRunLoopDoObservers | CA::Transaction::observer_callback(__CFRunLoopObserver*, unsigned long, void*) | CA::Transaction::commit() | CA::Context::commit_transaction(CA::Transaction*) | CALayerDisplayIfNeeded | -[CALayer _display] | CABackingStoreUpdate | backing_callback(CGContext*, void*) | -[CALayer drawInContext:] | -[UIView(CALayerDelegate) drawLayer:inContext:] | -[AvatarView drawRect:] | -[AvatarView overlayPNG:] | +[UIImageUtility createMaskOf:] | UIGraphicsGetImageFromCurrentImageContext | CGBitmapContextCreateImage | create_bitmap_data_provider | malloc | malloc_zone_malloc

and i really can't understand what i am doing wrong. here's the code of the [UIImageUtility createMaskOf:] function:

+ (UIImage *)createMaskOf:(UIImage *)source {
    CGRect rect = CGRectMake(0, 0, source.size.width, source.size.height);
    UIGraphicsBeginImageContext(CGSizeMake(source.size.width, source.size.height));
    CGContextRef context = UIGraphicsGetCurrentContext();

    CGContextTranslateCTM(context, 0, source.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);

    UIImage *original = [self createGrayCopy:source];

    CGContextRef context2 = CGBitmapContextCreate(NULL, source.size.width, source.size.height, 8, 4 * source.size.width, 
                                                   CGColorSpaceCreateDeviceRGB(), kCGImageAlphaNoneSkipLast);
    CGContextDrawImage(context2, CGRectMake(0, 0, source.size.width, source.size.height), original.CGImage);
    CGImageRef unmasked = CGBitmapContextCreateImage(context2);

    const float myMaskingColorsFrameColor[6] = { 1,256,1,256,1,256 };
    CGImageRef mask = CGImageCreateWithMaskingColors(unmasked, myMaskingColorsFrameColor);

    CGContextSetRGBFillColor (context, 256,256,256, 1);
    CGContextFillRect(context, rect);
    CGContextDrawImage(context, rect, mask);

    UIImage *whiteMasked = UIGraphicsGetImageFromCurrentImageContext();

    UIGraphicsEndImageContext();

    return whiteMasked;
}

the other custom function called before that is the following:

- (UIImage *)overlayPNG:(SinglePart *)sp {
    NSLog([sp description]);
    // Rect and context setup
    CGRect rect = CGRectMake(0, 0, sp.image.size.width, sp.image.size.height);
    NSLog(@"%f x %f", sp.image.size.width, sp.image.size.height);

    // Create an image of a color filled rectangle
    UIImage *baseColor = nil;
    if (sp.hasOwnColor) {
            baseColor = [UIImageUtility imageWithRect:rect ofColor:sp.color];
    } else {
        SinglePart *facePart = [editingAvatar.face.partList objectAtIndex:0];
        baseColor = [UIImageUtility imageWithRect:rect ofColor:facePart.color];
    }

    // Crete the mask of the layer
    UIImage *mask = [UIImageUtility createMaskOf:sp.image];
    mask = [UIImageUtility createGrayCopy:mask];

    // Create a new context for merging the overlay and a mask of the layer
    UIGraphicsBeginImageContext(CGSizeMake(sp.image.size.width, sp.image.size.height));
    CGContextRef context2 = UIGraphicsGetCurrentContext(); 

    // Adjust the coordinate system so that the origin 
    // is in the lower left corner of the view and the 
    // y axis points up 
    CGContextTranslateCTM(context2, 0, sp.image.size.height); 
    CGContextScaleCTM(context2, 1.0, -1.0); 

    // Create masked overlay color layer
    CGImageRef MaskedImage = CGImageCreateWithMask (baseColor.CGImage, mask.CGImage);

    // Draw the base color layer
    CGContextDrawImage(context2, rect, MaskedImage);

    // Get the result of the masking
    UIImage* overlayMasked = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    UIGraphicsBeginImageContext(CGSizeMake(sp.image.size.width, sp.image.size.height));
    CGContextRef context = UIGraphicsGetCurrentContext();

    // Adjust the coordinate system so that the origin 
    // is in the lower left corner of the view and the 
    // y axis points up 
    CGContextTranslateCTM(context, 0, sp.image.size.height);
    CGContextScaleCTM(context, 1.0, -1.0);

    // Get the result of the blending of the masked overlay and the base image
    CGContextDrawImage(context, rect, overlayMasked.CGImage);

    // Set the blend mode for the next drawn image
    CGContextSetBlendMode(context, kCGBlendModeOverlay);

    // Component image drawn
    CGContextDrawImage(context, rect, sp.image.CGImage);

    UIImage* blendedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    CGImageRelease(MaskedImage);

    return blendedImage;
}