diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index 4c3334f2e..a0a5b04ef 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -727,7 +727,6 @@ Naming/VariableNumber:
 # Cop supports --auto-correct.
 Performance/Casecmp:
   Exclude:
-    - 'lib/redmine/codeset_util.rb'
     - 'lib/redmine/scm/adapters/bazaar_adapter.rb'
 
 Performance/FixedSize:
diff --git a/lib/redmine/codeset_util.rb b/lib/redmine/codeset_util.rb
index 657d057d6..53728928c 100644
--- a/lib/redmine/codeset_util.rb
+++ b/lib/redmine/codeset_util.rb
@@ -22,7 +22,7 @@ module Redmine
         return str
       end
       enc = encoding.blank? ? "UTF-8" : encoding
-      if enc.upcase != "UTF-8"
+      if enc.casecmp("UTF-8") != 0
         str.force_encoding(enc)
         str = str.encode("UTF-8", :invalid => :replace,
               :undef => :replace, :replace => '?')
@@ -62,7 +62,7 @@ module Redmine
       str = str.dup
       str ||= ''
       str.force_encoding('UTF-8')
-      if encoding.upcase != 'UTF-8'
+      if encoding.casecmp('UTF-8') != 0
         str = str.encode(encoding, :invalid => :replace,
                          :undef => :replace, :replace => '?')
       else