diff --git a/README.md b/README.md
index 6c0618627..cae66f775 100644
--- a/README.md
+++ b/README.md
@@ -91,7 +91,7 @@ Procedures for each standard are provided by the following R7RS-style libraries:
 cmake -B build -DCMAKE_BUILD_TYPE=Release
 cd build
 make package
-sudo apt install build/meevax_0.5.77_amd64.deb
+sudo apt install build/meevax_0.5.78_amd64.deb
 ```
 
 or
@@ -123,9 +123,9 @@ sudo rm -rf /usr/local/share/meevax
 
 | Target Name | Description
 |-------------|-------------
-| `all`       | Build shared-library `libmeevax.0.5.77.so` and executable `meevax`
+| `all`       | Build shared-library `libmeevax.0.5.78.so` and executable `meevax`
 | `test`      | Test executable `meevax`
-| `package`   | Generate debian package `meevax_0.5.77_amd64.deb`
+| `package`   | Generate debian package `meevax_0.5.78_amd64.deb`
 | `install`   | Copy files into `/usr/local` directly
 
 ## Usage
diff --git a/VERSION b/VERSION
index c3ae0979f..4fbfa885b 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-0.5.77
+0.5.78
diff --git a/include/meevax/memory/gc_pointer.hpp b/include/meevax/memory/gc_pointer.hpp
index 3ce108e40..f139eadc2 100644
--- a/include/meevax/memory/gc_pointer.hpp
+++ b/include/meevax/memory/gc_pointer.hpp
@@ -47,8 +47,9 @@ inline namespace memory
     template <typename T, REQUIRES(std::is_scalar<T>)>
     explicit gc_pointer(T const& datum)
       : pointer { datum }
-      , collector::registration { locate(pointer::get()) }
-    {}
+    {
+      assert(pointer::get() == nullptr);
+    }
 
     auto operator =(gc_pointer const& gcp) -> auto &
     {
@@ -62,6 +63,12 @@ inline namespace memory
       return *this;
     }
 
+    auto operator =(std::nullptr_t) -> auto &
+    {
+      reset();
+      return *this;
+    }
+
     auto reset(gc_pointer const& gcp) -> void
     {
       pointer::reset(gcp);