diff --git a/source/dng_ifd.cpp b/source/dng_ifd.cpp
index 317ebf9..7f22065 100644
--- a/source/dng_ifd.cpp
+++ b/source/dng_ifd.cpp
@@ -3979,7 +3979,7 @@
 	if (fTileWidth)
 		{
 
-		return (fImageWidth + fTileWidth - 1) / fTileWidth;
+		return (SafeUint32Sub(SafeUint32Add(fImageWidth, fTileWidth), 1)) / fTileWidth;
 		
 		}
 		
@@ -3995,7 +3995,7 @@
 	if (fTileLength)
 		{
 
-		return (fImageLength + fTileLength - 1) / fTileLength;
+		return (SafeUint32Sub(SafeUint32Add(fImageLength, fTileLength), 1)) / fTileLength;
 		
 		}