diff --git a/lib/net/http/header.rb b/lib/net/http/header.rb
index a39043e..a597182 100644
--- a/lib/net/http/header.rb
+++ b/lib/net/http/header.rb
@@ -964,13 +964,13 @@ def set_form_data(params, sep = '&')
   #   Field names and values of non-file fields should be encoded with this charset.
   #
   def set_form(params, enctype='application/x-www-form-urlencoded', formopt={})
-    @body_data = params
-    @body = nil
-    @body_stream = nil
-    @form_option = formopt
     case enctype
     when /\Aapplication\/x-www-form-urlencoded\z/i,
       /\Amultipart\/form-data\z/i
+      @body_data = params
+      @body = nil
+      @body_stream = nil
+      @form_option = formopt
       self.content_type = enctype
     else
       raise ArgumentError, "invalid enctype: #{enctype}"