diff --git a/src/json.hpp b/src/json.hpp
index 565c0415f..9d2305d85 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -8991,7 +8991,7 @@ basic_json_parser_63:
         /// current level of recursion
         int depth = 0;
         /// callback function
-        const parser_callback_t callback;
+        const parser_callback_t callback = nullptr;
         /// the type of the last read token
         typename lexer::token_type last_token = lexer::token_type::uninitialized;
         /// the lexer
diff --git a/src/json.hpp.re2c b/src/json.hpp.re2c
index 26fafa543..537d4f4da 100644
--- a/src/json.hpp.re2c
+++ b/src/json.hpp.re2c
@@ -8288,7 +8288,7 @@ class basic_json
         /// current level of recursion
         int depth = 0;
         /// callback function
-        const parser_callback_t callback;
+        const parser_callback_t callback = nullptr;
         /// the type of the last read token
         typename lexer::token_type last_token = lexer::token_type::uninitialized;
         /// the lexer