diff --git a/ext/json/JSON_parser.c b/ext/json/JSON_parser.c
index dd2541bb7c9..2c4dd9a5f3f 100644
--- a/ext/json/JSON_parser.c
+++ b/ext/json/JSON_parser.c
@@ -29,6 +29,9 @@ SOFTWARE.
 #include <stdio.h>
 #include "JSON_parser.h"
 
+/* Windows defines IN for documentation */
+#undef IN
+
 #define true  1
 #define false 0
 #define __   -1     /* the universal error code */