diff --git a/pngpriv.h b/pngpriv.h
index 6d2e424d2..0b13740b0 100644
--- a/pngpriv.h
+++ b/pngpriv.h
@@ -466,7 +466,16 @@
    static_cast<type>(static_cast<const void*>(value))
 #else
 #  define png_voidcast(type, value) (value)
-#  define png_constcast(type, value) ((type)(value))
+#  ifdef _WIN64
+#     ifdef __GNUC__
+         typedef unsigned long long png_ptruint;
+#     else
+         typedef unsigned __int64 png_ptruint;
+#     endif
+#  else
+      typedef unsigned long png_ptruint;
+#  endif
+#  define png_constcast(type, value) ((type)(png_ptruint)(const void*)(value))
 #  define png_aligncast(type, value) ((void*)(value))
 #  define png_aligncastconst(type, value) ((const void*)(value))
 #endif /* __cplusplus */