diff --git a/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb b/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
index 452a456a52..ea4034303e 100644
--- a/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
+++ b/activesupport/lib/active_support/core_ext/module/attribute_accessors_per_thread.rb
@@ -53,8 +53,7 @@ class Module
         EOS
       end
 
-      default_val = (block_given? && default.nil?) ? yield : default
-      Thread.current["attr_" + name + "_#{sym}"] = default_val unless default_val.nil?
+      Thread.current["attr_" + name + "_#{sym}"] = default unless default.nil?
     end
   end
   alias :thread_cattr_reader :thread_mattr_reader
@@ -97,8 +96,7 @@ class Module
         EOS
       end
 
-      default_val = (block_given? && default.nil?) ? yield : default
-      Thread.current["attr_" + name + "_#{sym}"] = default_val unless default_val.nil?
+      public_send("#{sym}=", default) unless default.nil?
     end
   end
   alias :thread_cattr_writer :thread_mattr_writer
@@ -142,9 +140,9 @@ class Module
   #
   #   Current.new.user = "DHH"  # => NoMethodError
   #   Current.new.user          # => NoMethodError
-  def thread_mattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true, default: nil, &blk)
-    thread_mattr_reader(*syms, instance_reader: instance_reader, instance_accessor: instance_accessor, default: default, &blk)
-    thread_mattr_writer(*syms, instance_writer: instance_writer, instance_accessor: instance_accessor, default: default)
+  def thread_mattr_accessor(*syms, instance_reader: true, instance_writer: true, instance_accessor: true, default: nil)
+    thread_mattr_reader(*syms, instance_reader: instance_reader, instance_accessor: instance_accessor, default: default)
+    thread_mattr_writer(*syms, instance_writer: instance_writer, instance_accessor: instance_accessor)
   end
   alias :thread_cattr_accessor :thread_mattr_accessor
 end
diff --git a/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb b/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
index c73ca0be0c..a2b3239884 100644
--- a/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
+++ b/activesupport/test/core_ext/module/attribute_accessor_per_thread_test.rb
@@ -14,7 +14,7 @@ class ModuleAttributeAccessorPerThreadTest < ActiveSupport::TestCase
   class SubMyClass < MyClass
   end
 
-  def setup
+  setup do
     @class = MyClass
     @subclass = SubMyClass
     @object = @class.new
@@ -23,18 +23,11 @@ class ModuleAttributeAccessorPerThreadTest < ActiveSupport::TestCase
   def test_can_initialize_with_default_value
     Thread.new do
       @class.thread_mattr_accessor :baz, default: "default_value"
-      assert_equal "default_value", @class.baz
-    end.join
-  end
-
-  def test_can_initialize_with_a_block_as_default_value
-    Thread.new do
-      @class.thread_mattr_accessor :baz do
-        "default_value"
-      end
 
       assert_equal "default_value", @class.baz
     end.join
+
+    assert_nil @class.baz
   end
 
   def test_should_use_mattr_default
@@ -82,23 +75,23 @@ class ModuleAttributeAccessorPerThreadTest < ActiveSupport::TestCase
     threads = []
     threads << Thread.new do
       @class.foo = "things"
-      sleep 1
+      Thread.pass
       assert_equal "things", @class.foo
     end
 
     threads << Thread.new do
       @class.foo = "other things"
-      sleep 1
+      Thread.pass
       assert_equal "other things", @class.foo
     end
 
     threads << Thread.new do
       @class.foo = "really other things"
-      sleep 1
+      Thread.pass
       assert_equal "really other things", @class.foo
     end
 
-    threads.each { |t| t.join }
+    threads.each(&:join)
   end
 
   def test_should_raise_name_error_if_attribute_name_is_invalid