diff --git a/win32/Makefile.sub b/win32/Makefile.sub
index 7792d5d8e3..dba112259b 100644
--- a/win32/Makefile.sub
+++ b/win32/Makefile.sub
@@ -680,7 +680,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define _INTEGRAL_MAX_BITS 64
 #endif
 #define SIZEOF_OFF_T 8
-!if "$(ARCH)" == "x64"
+!if "$(TARGET_OS)" == "mswin64"
 #define SIZEOF_VOIDP 8
 !else
 #define SIZEOF_VOIDP 4
@@ -702,7 +702,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define NUM2CLOCKID(v) NUM2INT(v)
 #define SIZEOF_CLOCK_T 4
 #define SIZEOF_RLIM_T 0
-!if "$(ARCH)" == "x64"
+!if "$(TARGET_OS)" == "mswin64"
 #define SIZEOF_SIZE_T 8
 #define SIZEOF_PTRDIFF_T 8
 #define SIZEOF_INTPTR_T 8
@@ -802,7 +802,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define HAVE_INTPTR_T 1
 #define HAVE_UINTPTR_T 1
 #define HAVE_SSIZE_T 1
-!if "$(ARCH)" == "x64"
+!if "$(TARGET_OS)" == "mswin64"
 #define ssize_t __int64
 #define PRI_PTR_PREFIX "I64"
 !else