diff --git a/include/ruby/missing.h b/include/ruby/missing.h
index cda0aea164..68a90aeb96 100644
--- a/include/ruby/missing.h
+++ b/include/ruby/missing.h
@@ -185,6 +185,10 @@ RUBY_EXTERN int isnan(double);
 # endif
 #endif
 
+#ifndef HAVE_NAN
+RUBY_EXTERN double nan(const char *);
+#endif
+
 #ifndef HAVE_NEXTAFTER
 RUBY_EXTERN double nextafter(double x, double y);
 #endif
diff --git a/win32/Makefile.sub b/win32/Makefile.sub
index 73b26b3073..a853399d00 100644
--- a/win32/Makefile.sub
+++ b/win32/Makefile.sub
@@ -736,6 +736,9 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define HAVE_FLOCK 1
 #define HAVE_ISNAN 1
 #define HAVE_FINITE 1
+!if $(RT_VAR) >= 120
+#define HAVE_NAN 1
+!endif
 #define HAVE_HYPOT 1
 #define HAVE_FMOD 1
 #define HAVE_FREXP 1