execute addObject, but NSMutableArray has no object

I try to use to codes below to add object to NSMutableArray

NSMutableArray* multipartData;
- (void)processDataChunk:(NSData *)postDataChunk
{
    if (!postHeaderOK)
    {
        UInt16 separatorBytes = 0x0A0D;
        NSData* separatorData = [NSData dataWithBytes:&separatorBytes length:2];

        for (int i = 0; i < [postDataChunk length] - l; i++)
        {
            NSRange searchRange = {i, l};

            if ([[postDataChunk subdataWithRange:searchRange] isEqualToData:separatorData])
            {
                NSRange newDataRange = {dataStartIndex, i - dataStartIndex};
                dataStartIndex = i + l;
                i += l - 1;
                NSData *newData = [postDataChunk subdataWithRange:newDataRange];

                if ([newData length])
                {
                    [multipartData addObject:newData]; //A:set bre开发者_StackOverflow中文版ak point here
                }
            }
        }
    }
}

I set breakpoint at A:, and found that newData is not nil.

What is wrong with my codes?

Welcome any comment

---

You declare multipartData but do not allocate or initialize it. Somewhere before adding objects you must have

NSMutableArray *multipartData = [[NSMutableArray alloc] init];

or possibly

NSMutableArray *multipartData = [[[NSMutableArray alloc] init] autorelease];

as your needs dictate. But you must allocate and initialize multipartData before adding to or accessing anything in it.