How could this leak?

gistfile1.m

// How could this leaking?
// self.request is a ASIHttpRequest

[self.request setDataReceivedBlock: ^(NSData *receivedData) {
    NSAutoreleasePool *pool = [NSAutoreleasePool new];
    [intermediateData appendData: receivedData];
    if (CFAbsoluteTimeGetCurrent() - lastUpdateTime >= updateThreshold) {
        CGImageSourceUpdateData(partialImageSource, (CFDataRef)intermediateData, false);
        CGImageRef partialImage = NULL;
        partialImage = CGImageSourceCreateImageAtIndex(partialImageSource, 0, NULL); // Instruments say this line leaks,
        CGImageRelease(partialImage);                                                // But I release partialImage immediately. 
        lastUpdateTime = CFAbsoluteTimeGetCurrent();
    }
    [pool drain];
}];

/* Instrument Leak Report

Leaked Object	#	Address	Size	Responsible Library	Responsible Frame
,12	< multiple >	192 Bytes	ImageIO	readPropLong
,8	< multiple >	128 Bytes	ImageIO	readPropRtnl
NSCFString,8	< multiple >	320 Bytes	ImageIO	readProps
Malloc 16 Bytes,6	< multiple >	96 Bytes	ImageIO	readPropShort
?,6	< multiple >	288 Bytes	ImageIO	CGImagePlusCreateWithSession
Malloc 64 Bytes,6	< multiple >	384 Bytes	ImageIO	CGImagePlusSetGeometry
,6	< multiple >	672 Bytes	ImageIO	CGImagePlusCreateWithSession
Malloc 64 Bytes,6	< multiple >	384 Bytes	ImageIO	CGImagePlusSetGeometry
Malloc 16 Bytes,6	< multiple >	96 Bytes	ImageIO	readPropShort
?,6	< multiple >	288 Bytes	ImageIO	readPropShort
,6	< multiple >	96 Bytes	ImageIO	CGImagePlusSetGeometry
,6	< multiple >	96 Bytes	ImageIO	CGImagePlusSetGeometry
,6	< multiple >	96 Bytes	ImageIO	readPropShort
,6	< multiple >	96 Bytes	ImageIO	copyTiffPropertiesToRoot
,5	< multiple >	80 Bytes	ImageIO	CGImageReadSessionCreate
?,5	< multiple >	240 Bytes	ImageIO	readJFIFData
Malloc 96 Bytes,5	< multiple >	480 Bytes	ImageIO	readPropShort
Malloc 32 Bytes,5	< multiple >	160 Bytes	ImageIO	readPropShort
Malloc 96 Bytes,5	< multiple >	480 Bytes	ImageIO	readPropShort
Malloc 96 Bytes,5	< multiple >	480 Bytes	ImageIO	CGImagePlusCreateWithSession
?,5	< multiple >	240 Bytes	ImageIO	readPropShort
Malloc 96 Bytes,5	< multiple >	480 Bytes	ImageIO	readJFIFData
,5	< multiple >	80 Bytes	ImageIO	copyTiffPropertiesToRoot
Malloc 32 Bytes,4	< multiple >	128 Bytes	ImageIO	readJFIFData
Malloc 48 Bytes,4	< multiple >	192 Bytes	ImageIO	readJFIFData
,4	< multiple >	704 Bytes	ImageIO	CGImageReadCreateWithData
,4	< multiple >	128 Bytes	ImageIO	readJFIFData
Malloc 32 Bytes,4	< multiple >	128 Bytes	ImageIO	readJFIFData
Malloc 32 Bytes,4	< multiple >	128 Bytes	ImageIO	readPropShort
,4	< multiple >	64 Bytes	ImageIO	readJFIFData
,4	< multiple >	64 Bytes	ImageIO	readJFIFData


*/