diff --git a/actionmailer/lib/action_mailer/old_api.rb b/actionmailer/lib/action_mailer/old_api.rb
index a9cdf6c0ca..0396f0775e 100644
--- a/actionmailer/lib/action_mailer/old_api.rb
+++ b/actionmailer/lib/action_mailer/old_api.rb
@@ -248,7 +248,7 @@ module ActionMailer
       else
         ctype, *attrs = @content_type.split(/;\s*/)
         attrs = Hash[attrs.map { |attr| attr.split(/\=/, 2) }]
-        [ctype, {"charset" => @charset}.merge(attrs)]
+        [ctype, {"charset" => @charset}.merge!(attrs)]
       end
     end
   end