diff --git a/bootstraptest/test_ractor.rb b/bootstraptest/test_ractor.rb
index 51d2762d939..cedc73da789 100644
--- a/bootstraptest/test_ractor.rb
+++ b/bootstraptest/test_ractor.rb
@@ -888,6 +888,91 @@ assert_equal "can not get unshareable values from instance variables of classes/
   end
 RUBY
 
+# setting an ivar of a class/module from a non-owner Ractor reports the ivar and class
+assert_equal "can not set instance variables of classes/modules created by another Ractor (@iv from C)", <<~'RUBY', frozen_string_literal: false
+  class C
+    @iv = 'str'
+  end
+
+  r = Ractor.new do
+    class C
+      @iv = 'other'
+    end
+  end
+
+  begin
+    r.value
+  rescue Ractor::RemoteError => e
+    e.cause.message
+  end
+RUBY
+
+# a non-main Ractor can read, set, and remove ivars of classes/modules it created
+assert_equal 'ok', <<~'RUBY', frozen_string_literal: false
+  Ractor.new do
+    [Class.new, Module.new].each do |mod|
+      mod.instance_variable_set(:@iv, 'str')
+      raise unless mod.instance_variable_get(:@iv) == 'str'
+      raise unless mod.remove_instance_variable(:@iv) == 'str'
+    end
+    :ok
+  end.value
+RUBY
+
+# even main cannot modify another Ractor's module; reporting its name must not call to_s
+assert_equal 'true', <<~'RUBY', frozen_string_literal: false
+  port = Ractor::Port.new
+  r = Ractor.new(port) do |port|
+    mod = Module.new
+    mod.set_temporary_name('worker_module')
+    mod.instance_variable_set(:@iv, 'str')
+    def mod.to_s = raise('to_s must not be called')
+    port << mod
+    Ractor.receive
+  end
+  mod = port.receive
+
+  begin
+    messages = [[:instance_variable_set, :@iv, 'other'], [:remove_instance_variable, :@iv]].map do |args|
+      begin
+        mod.public_send(*args)
+      rescue Ractor::IsolationError => e
+        e.message
+      end
+    end
+    messages == ["can not set instance variables of classes/modules created by another Ractor (@iv from worker_module)"] * 2
+  ensure
+    r << nil
+    r.join
+  end
+RUBY
+
+# normal and JIT-compiled ivar reads report the same names without calling to_s
+assert_equal 'true', <<~'RUBY', frozen_string_literal: false
+  class C
+    @iv = 'str'
+    def self.read = @iv
+    def self.to_s = raise('to_s must not be called')
+  end
+
+  Ractor.new do
+    messages = []
+    begin
+      C.instance_variable_get(:@iv)
+    rescue Ractor::IsolationError => e
+      messages << e.message
+    end
+    40.times do
+      begin
+        C.read
+      rescue Ractor::IsolationError => e
+        messages << e.message
+      end
+    end
+    messages == ["can not get unshareable values from instance variables of classes/modules created by another Ractor (@iv from C)"] * 41
+  end.value
+RUBY
+
 # setting an ivar on a shareable but unfrozen object is not allowed, by instance_variable_set
 assert_equal "can't modify instance variables of a shareable Ractor", %q{
   shared = Ractor.new{}
diff --git a/variable.c b/variable.c
index 5d6e1b17637..e23c5056f51 100644
--- a/variable.c
+++ b/variable.c
@@ -1274,7 +1274,13 @@ class_ivar_set_ractor_check(VALUE klass, ID id)
 {
     if (rb_is_instance_id(id) && // check only normal ivars
         UNLIKELY(!rb_class_owned_p(klass))) {
-        rb_raise(rb_eRactorIsolationError, "can not set instance variables of classes/modules created by another Ractor");
+        rb_raise(
+            rb_eRactorIsolationError,
+            "can not set instance variables of classes/modules "
+            "created by another Ractor (%"PRIsVALUE" from %"PRIsVALUE")",
+            rb_id2str(id),
+            rb_class_path(klass)
+        );
     }
 }
 
@@ -1627,7 +1633,7 @@ rb_ivar_lookup(VALUE obj, ID id, VALUE undef)
                 "can not get unshareable values from instance variables of classes/modules "
                 "created by another Ractor (%"PRIsVALUE" from %"PRIsVALUE")",
                 rb_id2str(id),
-                obj
+                rb_class_path(obj)
             );
         }
     }
@@ -1659,8 +1665,13 @@ rb_ivar_get_at(VALUE obj, attr_index_t index, ID id)
             VALUE val = rb_imemo_fields_ptr(fields_obj)[index];
 
             if (UNLIKELY(!rb_class_owned_p(obj)) && !rb_ractor_shareable_p(val)) {
-                rb_raise(rb_eRactorIsolationError,
-                        "can not get unshareable values from instance variables of classes/modules created by another Ractor");
+                rb_raise(
+                    rb_eRactorIsolationError,
+                    "can not get unshareable values from instance variables of classes/modules "
+                    "created by another Ractor (%"PRIsVALUE" from %"PRIsVALUE")",
+                    rb_id2str(id),
+                    rb_class_path(obj)
+                );
             }
 
             return val;