diff --git a/DirectXTex/d3dx12.h b/DirectXTex/d3dx12.h
index 45f6b4e..8016989 100644
--- a/DirectXTex/d3dx12.h
+++ b/DirectXTex/d3dx12.h
@@ -1868,12 +1868,12 @@ inline void MemcpySubresource(
 {
     for (UINT z = 0; z < NumSlices; ++z)
     {
-        BYTE* pDestSlice = reinterpret_cast<BYTE*>(pDest->pData) + pDest->SlicePitch * z;
-        const BYTE* pSrcSlice = reinterpret_cast<const BYTE*>(pSrc->pData) + pSrc->SlicePitch * z;
+        auto pDestSlice = reinterpret_cast<BYTE*>(pDest->pData) + pDest->SlicePitch * z;
+        auto pSrcSlice = reinterpret_cast<const BYTE*>(pSrc->pData) + pSrc->SlicePitch * LONG_PTR(z);
         for (UINT y = 0; y < NumRows; ++y)
         {
             memcpy(pDestSlice + pDest->RowPitch * y,
-                   pSrcSlice + pSrc->RowPitch * y,
+                   pSrcSlice + pSrc->RowPitch * LONG_PTR(y),
                    RowSizeInBytes);
         }
     }