diff --git a/src/osx/core/bitmap.cpp b/src/osx/core/bitmap.cpp
index fcfb40816b..004b67c347 100644
--- a/src/osx/core/bitmap.cpp
+++ b/src/osx/core/bitmap.cpp
@@ -1026,8 +1026,8 @@ wxBitmap wxBitmap::GetSubBitmap(const wxRect &rect) const
         {
             int sourcelinesize = GetBitmapData()->GetBytesPerRow() ;
             int destlinesize = ret.GetBitmapData()->GetBytesPerRow() ;
-            unsigned char *source = sourcedata + int(rect.x * scale * 4 + rect.y *scale * sourcelinesize) ;
-            unsigned char *dest = destdata ;
+            unsigned char* source = sourcedata + size_t(rect.x * scale) * 4 + size_t(rect.y * scale) * sourcelinesize;
+            unsigned char* dest = destdata;
 
             for (int yy = 0; yy < destheight; ++yy, source += sourcelinesize , dest += destlinesize)
             {