diff --git a/json11.cpp b/json11.cpp
index 07e43c0..3ec83af 100644
--- a/json11.cpp
+++ b/json11.cpp
@@ -338,7 +338,7 @@ struct JsonParser {
     size_t i;
     string &err;
     bool failed;
-    JsonParse strategy;
+    const JsonParse strategy;
 
     /* fail(msg, err_ret = Json())
      *