diff --git a/src/json.hpp b/src/json.hpp
index 459f7f3d..bc6fd709 100644
--- a/src/json.hpp
+++ b/src/json.hpp
@@ -5967,7 +5967,7 @@ class basic_json
                         {
                             return (v < 10)
                             ? ('0' + static_cast<char>(v))
-                            : ('a' + static_cast<char>((v - 10) & 0xff));
+                            : ('a' + static_cast<char>((v - 10) & 0x1f));
                         };
 
                         // print character c as \uxxxx
diff --git a/src/json.hpp.re2c b/src/json.hpp.re2c
index 685024a5..793cc738 100644
--- a/src/json.hpp.re2c
+++ b/src/json.hpp.re2c
@@ -5967,7 +5967,7 @@ class basic_json
                         {
                             return (v < 10)
                             ? ('0' + static_cast<char>(v))
-                            : ('a' + static_cast<char>((v - 10) & 0xff));
+                            : ('a' + static_cast<char>((v - 10) & 0x1f));
                         };
 
                         // print character c as \uxxxx