diff --git a/mrbgems/mruby-symbol-ext/mrblib/symbol.rb b/mrbgems/mruby-symbol-ext/mrblib/symbol.rb
index 4cf18f647..b2615a760 100644
--- a/mrbgems/mruby-symbol-ext/mrblib/symbol.rb
+++ b/mrbgems/mruby-symbol-ext/mrblib/symbol.rb
@@ -25,7 +25,7 @@ class Symbol
   # Same as <code>sym.to_s.capitalize.intern</code>.
 
   def capitalize
-    self.to_s.capitalize.intern
+    (self.to_s.capitalize! || self).to_sym
   end
 
   ##
@@ -35,7 +35,7 @@ class Symbol
   # Same as <code>sym.to_s.downcase.intern</code>.
 
   def downcase
-    self.to_s.downcase.intern
+    (self.to_s.downcase! || self).to_sym
   end
 
   ##
@@ -45,7 +45,7 @@ class Symbol
   # Same as <code>sym.to_s.upcase.intern</code>.
 
   def upcase
-    self.to_s.upcase.intern
+    (self.to_s.upcase! || self).to_sym
   end
 
   ##
diff --git a/mrbgems/mruby-symbol-ext/test/symbol.rb b/mrbgems/mruby-symbol-ext/test/symbol.rb
index 59df7ea9d..c2695f1f8 100644
--- a/mrbgems/mruby-symbol-ext/test/symbol.rb
+++ b/mrbgems/mruby-symbol-ext/test/symbol.rb
@@ -19,14 +19,17 @@ end
 assert("Symbol#capitalize") do
   assert_equal :Hello, :hello.capitalize
   assert_equal :Hello, :HELLO.capitalize
+  assert_equal :Hello, :Hello.capitalize
 end
 
 assert("Symbol#downcase") do
   assert_equal :hello, :hEllO.downcase
+  assert_equal :hello, :hello.downcase
 end
 
 assert("Symbol#upcase") do
   assert_equal :HELLO, :hEllO.upcase
+  assert_equal :HELLO, :HELLO.upcase
 end
 
 assert("Symbol#casecmp") do