diff --git a/README.md b/README.md
index 4b368be..1bad6dd 100644
--- a/README.md
+++ b/README.md
@@ -19,7 +19,7 @@ gem install mailgun-ruby
 Gemfile:
 
 ```ruby
-gem 'mailgun-ruby', '~>1.2.3'
+gem 'mailgun-ruby', '~>1.2.4'
 ```
 
 Usage
diff --git a/lib/mailgun/client.rb b/lib/mailgun/client.rb
index b25557d..940ac3c 100755
--- a/lib/mailgun/client.rb
+++ b/lib/mailgun/client.rb
@@ -64,6 +64,8 @@ def self.deliveries
     # containing required parameters for the requested resource.
     # @return [Mailgun::Response] A Mailgun::Response object.
     def send_message(working_domain, data)
+      fail ParameterError.new('Missing working domain', working_domain) unless working_domain
+
       if test_mode? then
         Mailgun::Client.deliveries << data
         return Response.from_hash(
diff --git a/lib/mailgun/messages/message_builder.rb b/lib/mailgun/messages/message_builder.rb
index a45cafc..562b4f9 100644
--- a/lib/mailgun/messages/message_builder.rb
+++ b/lib/mailgun/messages/message_builder.rb
@@ -270,8 +270,12 @@ def set_custom_data(name, data)
     # @return [void]
     def variable(name, data)
       fail(Mailgun::ParameterError, 'Variable name must be specified') if name.to_s.empty?
-      jsondata = make_json data
-      set_single("v:#{name}", jsondata)
+      begin
+        jsondata = make_json data
+        set_single("v:#{name}", jsondata)
+      rescue Mailgun::ParameterError
+        set_single("v:#{name}", data)
+      end
     end
 
     # Add custom parameter to the message. A custom parameter is any parameter that
diff --git a/lib/mailgun/version.rb b/lib/mailgun/version.rb
index ac8a140..08091cf 100644
--- a/lib/mailgun/version.rb
+++ b/lib/mailgun/version.rb
@@ -1,4 +1,4 @@
 # It's the version. Yeay!
 module Mailgun
-  VERSION = '1.2.3'
+  VERSION = '1.2.4'
 end
diff --git a/lib/railgun/mailer.rb b/lib/railgun/mailer.rb
index b13a129..446fee2 100644
--- a/lib/railgun/mailer.rb
+++ b/lib/railgun/mailer.rb
@@ -11,7 +11,7 @@ module Railgun
   class Mailer
 
     # List of the headers that will be ignored when copying headers from `mail.header_fields`
-    IGNORED_HEADERS = %w[ to from subject reply-to ]
+    IGNORED_HEADERS = %w[ to from subject reply-to mime-version ]
 
     # [Hash] config ->
     #   Requires *at least* `api_key` and `domain` keys.
diff --git a/lib/railgun/railtie.rb b/lib/railgun/railtie.rb
index 3c91a35..a8b7107 100644
--- a/lib/railgun/railtie.rb
+++ b/lib/railgun/railtie.rb
@@ -2,8 +2,9 @@
 
 module Railgun
   class Railtie < ::Rails::Railtie
-    config.before_configuration do
-      ActionMailer::Base.add_delivery_method :mailgun, Railgun::Mailer
+    ActiveSupport.on_load(:action_mailer) do
+      add_delivery_method :mailgun, Railgun::Mailer
+      ActiveSupport.run_load_hooks(:mailgun_mailer, Railgun::Mailer)
     end
   end
 end
diff --git a/spec/unit/messages/message_builder_spec.rb b/spec/unit/messages/message_builder_spec.rb
index fb2810a..e35e1ce 100644
--- a/spec/unit/messages/message_builder_spec.rb
+++ b/spec/unit/messages/message_builder_spec.rb
@@ -567,9 +567,13 @@
     expect(@mb_obj.message["v:my-data"]).to be_kind_of(String)
     expect(@mb_obj.message["v:my-data"].to_s).to eq('{"key":"value"}')
   end
-  it 'throws an exception on broken JSON.' do
-    data = 'This is some crappy JSON.'
-    expect {@mb_obj.variable('my-data', data)}.to raise_error(Mailgun::ParameterError)
+  it 'accepts string values' do
+    data = 'String Value.'
+
+    @mb_obj.variable('my-data', data)
+
+    expect(@mb_obj.message["v:my-data"]).to be_kind_of(String)
+    expect(@mb_obj.message["v:my-data"].to_s).to eq('String Value.')
   end
 end
 
diff --git a/spec/unit/railgun/mailer_spec.rb b/spec/unit/railgun/mailer_spec.rb
index 3476354..525bb1c 100644
--- a/spec/unit/railgun/mailer_spec.rb
+++ b/spec/unit/railgun/mailer_spec.rb
@@ -191,6 +191,19 @@ def message_with_attachment(address, subject)
     expect(body['h:reply-to']).to eq('dude@example.com.au')
   end
 
+  it 'ignores `mime-version` in headers' do
+    message = UnitTestMailer.plain_message('test@example.org', '', {
+      'mime-version' => '1.0',
+    })
+    message.mailgun_headers = {
+      'Mime-Version' => '1.1',
+    }
+    message.headers({'MIME-VERSION' => '1.2'})
+
+    body = Railgun.transform_for_mailgun(message)
+    expect(body).not_to include('h:mime-version')
+  end
+
   it 'treats `headers()` names as case-insensitve' do
     message = UnitTestMailer.plain_message('test@example.org', '', {
       'X-BIG-VALUE' => 1,