ios – UIActivityViewController无法将图像添加到imessage

我正在使用UIActivityViewController在ios 6和我的代码上共享图像

UIImage *shareImage = [UIImage imagenamed:@"myImage.png"];

        // Init array
        NSMutableArray *items = [NSMutableArray new];
        [items addObject:@""];
        [items addObject:shareImage];

        // Add image to paste board
        UIPasteboard *pasteboard = [UIPasteboard generalPasteboard];
        pasteboard.persistent = YES;
        pasteboard.image = shareImage;

        UIActivityViewController *activityVC = [[UIActivityViewController alloc]initWithActivityItems:items applicationActivities:nil];

        //This is an array of excluded activities to appear on the UIActivityViewController
        activityVC.excludedActivityTypes = @[UIActivityTypePostToWeibo,UIActivityTypePrint,UIActivityTypeCopyToPasteboard,UIActivityTypeAssignToContact,UIActivityTypeSaveToCameraRoll];

        [self presentViewController:activityVC animated:TRUE completion:nil];

一切正常但无法将shareImage粘贴到imessage.我不知道为什么？
谢谢您帮忙！