--- a/image/SurfaceCache.cpp
+++ b/image/SurfaceCache.cpp
@@ -322,17 +322,18 @@ public:
         continue;
       }
 
       SurfaceKey bestMatchKey = bestMatch->GetSurfaceKey();
 
       // Compare sizes. We use an area-based heuristic here instead of computing a
       // truly optimal answer, since it seems very unlikely to make a difference
       // for realistic sizes.
-      int64_t idealArea = idealKey.Size().width * idealKey.Size().height;
+      int64_t idealArea = static_cast<int64_t>(idealKey.Size().width) *
+        static_cast<int64_t>(idealKey.Size().height);
       int64_t surfaceArea = aSurfaceKey.Size().width * aSurfaceKey.Size().height;
       int64_t bestMatchArea =
         bestMatchKey.Size().width * bestMatchKey.Size().height;
 
       // If the best match is smaller than the ideal size, prefer bigger sizes.
       if (bestMatchArea < idealArea) {
         if (surfaceArea > bestMatchArea) {
           bestMatch = surface;