diff --git a/README.md b/README.md
index ba8f129fc..69fda70f7 100644
--- a/README.md
+++ b/README.md
@@ -74,7 +74,7 @@ Subset of R7RS-small.
 cmake -B build -DCMAKE_BUILD_TYPE=Release
 cd build
 make package
-sudo apt install build/meevax_0.4.818_amd64.deb
+sudo apt install build/meevax_0.4.821_amd64.deb
 ```
 
 or
@@ -106,9 +106,9 @@ sudo rm -rf /usr/local/share/meevax
 
 | Target Name | Description
 |-------------|-------------
-| `all`       | Build shared-library `libmeevax.0.4.818.so` and executable `meevax`
+| `all`       | Build shared-library `libmeevax.0.4.821.so` and executable `meevax`
 | `test`      | Test executable `meevax`
-| `package`   | Generate debian package `meevax_0.4.818_amd64.deb`
+| `package`   | Generate debian package `meevax_0.4.821_amd64.deb`
 | `install`   | Copy files into `/usr/local` directly
 
 ## Usage
diff --git a/VERSION b/VERSION
index e0f4e0ff5..970e31c46 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.4.818
+0.4.821
diff --git a/include/meevax/memory/nan_boxing_pointer.hpp b/include/meevax/memory/nan_boxing_pointer.hpp
index b02a9be20..437e40f40 100644
--- a/include/meevax/memory/nan_boxing_pointer.hpp
+++ b/include/meevax/memory/nan_boxing_pointer.hpp
@@ -195,11 +195,11 @@ inline namespace memory
     {
       switch (signature())
       {
-      #define DEFINE(TYPE)                                                     \
+      #define CASE(TYPE)                                                       \
       case signature_##TYPE:                                                   \
         if constexpr (std::is_same_v<TYPE, bool>)                              \
         {                                                                      \
-          return os << std::boolalpha << yellow('#', as<TYPE>());              \
+          return os << yellow('#', as<TYPE>() ? 't' : 'f');                    \
         }                                                                      \
         else                                                                   \
         {                                                                      \
@@ -207,15 +207,15 @@ inline namespace memory
         }                                                                      \
         static_assert(true)
 
-      DEFINE(T1);
-      DEFINE(T2);
-      DEFINE(T3);
-      DEFINE(T4);
-      DEFINE(T5);
-      DEFINE(T6);
-      DEFINE(pointer);
+      CASE(T1);
+      CASE(T2);
+      CASE(T3);
+      CASE(T4);
+      CASE(T5);
+      CASE(T6);
+      CASE(pointer);
 
-      #undef DEFINE
+      #undef CASE
 
       default:
         if (auto value = as<double>(); std::isnan(value))