diff --git a/reactos/include/psdk/winnt.h b/reactos/include/psdk/winnt.h
index 039cf6073a0..3f7111ac5c3 100644
--- a/reactos/include/psdk/winnt.h
+++ b/reactos/include/psdk/winnt.h
@@ -1480,6 +1480,7 @@ typedef enum {
 #define MAXBYTE	0xff
 #define MAXWORD	0xffff
 #define MAXDWORD	0xffffffff
+#define MAXLONGLONG (((LONGLONG)0x7fffffff << 32) | 0xffffffff)
 #define PROCESSOR_INTEL_386 386
 #define PROCESSOR_INTEL_486 486
 #define PROCESSOR_INTEL_PENTIUM 586