diff --git a/configure.ac b/configure.ac
index 0e22aabbe5..711ff997e7 100644
--- a/configure.ac
+++ b/configure.ac
@@ -785,7 +785,8 @@ AS_IF([test "$GCC" = yes], [
 				 [disable -D_FORTIFY_SOURCE=2 option, which causes link error on mingw]),
 		  [fortify_source=$enableval])
     AS_IF([test "x$fortify_source" != xno], [
-        RUBY_TRY_CFLAGS([$optflags -D_FORTIFY_SOURCE=2], [RUBY_APPEND_OPTION(XCFLAGS, -D_FORTIFY_SOURCE=2)], [],
+        RUBY_TRY_CFLAGS([$optflags -D_FORTIFY_SOURCE=2],
+                        [RUBY_APPEND_OPTION(XCFLAGS, -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2)], [],
                         [@%:@include <stdio.h>])
     ])