diff --git a/NEWS.md b/NEWS.md
index 4b16fd50189d0b..465f8ec4b204a8 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -456,6 +456,7 @@ A lot of work has gone into making Ractors more stable, performant, and usable.
 [Feature #21205]: https://bugs.ruby-lang.org/issues/21205
 [Feature #21216]: https://bugs.ruby-lang.org/issues/21216
 [Feature #21219]: https://bugs.ruby-lang.org/issues/21219
+[Feature #21254]: https://bugs.ruby-lang.org/issues/21254
 [Feature #21258]: https://bugs.ruby-lang.org/issues/21258
 [Feature #21262]: https://bugs.ruby-lang.org/issues/21262
 [Feature #21275]: https://bugs.ruby-lang.org/issues/21275
diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index 6b35bbb46be896..575b96e48d501a 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -846,7 +846,7 @@ def ractor_local_globals
 }
 
 # ivar in shareable-objects are not allowed to access from non-main Ractor
-assert_equal "can not get unshareable values from instance variables of classes/modules from non-main Ractors", <<~'RUBY', frozen_string_literal: false
+assert_equal "can not get unshareable values from instance variables of classes/modules from non-main Ractors (@iv from C)", <<~'RUBY', frozen_string_literal: false
   class C
     @iv = 'str'
   end
@@ -1022,7 +1022,7 @@ def initialize
 }
 
 # cvar in shareable-objects are not allowed to access from non-main Ractor
-assert_equal 'can not access class variables from non-main Ractors', %q{
+assert_equal 'can not access class variables from non-main Ractors (@@cv from C)', %q{
   class C
     @@cv = 'str'
   end
@@ -1041,7 +1041,7 @@ class C
 }
 
 # also cached cvar in shareable-objects are not allowed to access from non-main Ractor
-assert_equal 'can not access class variables from non-main Ractors', %q{
+assert_equal 'can not access class variables from non-main Ractors (@@cv from C)', %q{
   class C
     @@cv = 'str'
     def self.cv
diff --git a/man/ruby.1 b/man/ruby.1
index d19cea99ca3c5b..8141a6d7efa97a 100644
--- a/man/ruby.1
+++ b/man/ruby.1
@@ -523,6 +523,13 @@ variable is not defined, Ruby refers to
 If set, Ruby tries to free all dynamically allocated memories.
 Introduced in Ruby 3.3, default: unset.
 .Pp
+.It Ev RUBY_BOX
+If set to
+.Li 1 ,
+Ruby Box is enabled and users will be able to execute
+.Li Ruby::Box.new .
+Ruby Box is an experimental feature introduced in Ruby 4.0.
+.Pp
 .It Ev RUBY_IO_BUFFER_DEFAULT_SIZE
 The custom default buffer size of
 .Li IO::Buffer .
diff --git a/string.c b/string.c
index 060a1467a06f55..9819a5910fa76e 100644
--- a/string.c
+++ b/string.c
@@ -716,7 +716,7 @@ VALUE rb_fs;
 static inline const char *
 search_nonascii(const char *p, const char *e)
 {
-    const uintptr_t *s, *t;
+    const char *s, *t;
 
 #if defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L)
 # if SIZEOF_UINTPTR_T == 8
@@ -760,17 +760,19 @@ search_nonascii(const char *p, const char *e)
 #define aligned_ptr(value) \
         __builtin_assume_aligned((value), sizeof(uintptr_t))
 #else
-#define aligned_ptr(value) (uintptr_t *)(value)
+#define aligned_ptr(value) (value)
 #endif
         s = aligned_ptr(p);
-        t = (uintptr_t *)(e - (SIZEOF_VOIDP-1));
+        t = (e - (SIZEOF_VOIDP-1));
 #undef aligned_ptr
-        for (;s < t; s++) {
-            if (*s & NONASCII_MASK) {
+        for (;s < t; s += sizeof(uintptr_t)) {
+            uintptr_t word;
+            memcpy(&word, s, sizeof(word));
+            if (word & NONASCII_MASK) {
 #ifdef WORDS_BIGENDIAN
-                return (const char *)s + (nlz_intptr(*s&NONASCII_MASK)>>3);
+                return (const char *)s + (nlz_intptr(word&NONASCII_MASK)>>3);
 #else
-                return (const char *)s + (ntz_intptr(*s&NONASCII_MASK)>>3);
+                return (const char *)s + (ntz_intptr(word&NONASCII_MASK)>>3);
 #endif
             }
         }
diff --git a/variable.c b/variable.c
index 47b521856676dd..7a015a51e397e0 100644
--- a/variable.c
+++ b/variable.c
@@ -1195,10 +1195,13 @@ IVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(ID id)
     }
 }
 
-#define CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR() \
-  if (UNLIKELY(!rb_ractor_main_p())) { \
-      rb_raise(rb_eRactorIsolationError, "can not access class variables from non-main Ractors"); \
-  }
+static void
+CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(VALUE klass, ID id)
+{
+    if (UNLIKELY(!rb_ractor_main_p())) {
+        rb_raise(rb_eRactorIsolationError, "can not access class variables from non-main Ractors (%"PRIsVALUE" from %"PRIsVALUE")", rb_id2str(id), klass);
+    }
+}
 
 static inline void
 ivar_ractor_check(VALUE obj, ID id)
@@ -1444,7 +1447,8 @@ rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
                     UNLIKELY(!rb_ractor_main_p()) &&
                     !rb_ractor_shareable_p(val)) {
                 rb_raise(rb_eRactorIsolationError,
-                        "can not get unshareable values from instance variables of classes/modules from non-main Ractors");
+                        "can not get unshareable values from instance variables of classes/modules from non-main Ractors (%"PRIsVALUE" from %"PRIsVALUE")",
+                        rb_id2str(id), obj);
             }
             return val;
         }
@@ -4202,7 +4206,7 @@ cvar_overtaken(VALUE front, VALUE target, ID id)
     }
 
 #define CVAR_LOOKUP(v,r) do {\
-    CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(); \
+    CVAR_ACCESSOR_SHOULD_BE_MAIN_RACTOR(klass, id); \
     if (cvar_lookup_at(klass, id, (v))) {r;}\
     CVAR_FOREACH_ANCESTORS(klass, v, r);\
 } while(0)