Unsupported image format for tiff image loaded using QImage

I've been using 4.6.3 on Mac OS X 10.6.5 without issue, and then when I upgraded to Qt 4.7.1 I find I cannot load a tiff image. The error reported by QImageReader is "Unsupported Image Format," but the image loaded in 4.6.3 without any problems. I've looked at the file settings using tiffdump(), but I don't see anything out of the ordinary.

Here is my source code:

for (int i=0; i<10; i++){
QString filestring=QString(":/Masks/Data/mask0%1.tif").arg;
QImageReader imageReader(filestring, "TIF");
QImage maskImage=imageReader.read();
if (maskImage.isNull())

{ QMessageBox::critical(0, QString("Initialize Masks"), imageReader.errorString(), ... }

}