--- a/gfx/2d/QuartzSupport.mm
+++ b/gfx/2d/QuartzSupport.mm
@@ -935,18 +935,18 @@ nsresult nsCARenderer::DrawSurfaceToCGCo
 
   // We get rendering glitches if we use a width/height that falls
   // outside of the IOSurface.
   if (aWidth + aX > ioWidth) 
     aWidth = ioWidth - aX;
   if (aHeight + aY > ioHeight) 
     aHeight = ioHeight - aY;
 
-  if (aX < 0 || aX >= ioWidth ||
-      aY < 0 || aY >= ioHeight) {
+  if (aX < 0 || static_cast<size_t>(aX) >= ioWidth ||
+      aY < 0 || static_cast<size_t>(aY) >= ioHeight) {
     surf->Unlock();
     return NS_ERROR_FAILURE;
   }
 
   void* ioData = surf->GetBaseAddress();
   double scaleFactor = surf->GetContentsScaleFactor();
   size_t intScaleFactor = ceil(surf->GetContentsScaleFactor());
   CGDataProviderRef dataProvider = ::CGDataProviderCreateWithData(ioData,