diff --git a/lib/net/http/header.rb b/lib/net/http/header.rb
index a8901e7..3f1a008 100644
--- a/lib/net/http/header.rb
+++ b/lib/net/http/header.rb
@@ -475,9 +475,8 @@ def set_form(params, enctype='application/x-www-form-urlencoded', formopt={})
     @body = nil
     @body_stream = nil
     @form_option = formopt
-    case enctype
-    when /\Aapplication\/x-www-form-urlencoded\z/i,
-      /\Amultipart\/form-data\z/i
+    case enctype.downcase
+    when 'application/x-www-form-urlencoded', 'multipart/form-data'
       self.content_type = enctype
     else
       raise ArgumentError, "invalid enctype: #{enctype}"