diff --git a/src/json.hpp b/src/json.hpp
index 146b52867..b4dc422d8 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -12529,7 +12529,7 @@ scan_number_done:
         }
 
         /// return syntax error message
-        const char* get_error_message() const noexcept
+        constexpr const char* get_error_message() const noexcept
         {
             return error_message;
         }