diff --git a/lib/govuk_design_system_formbuilder/elements/caption.rb b/lib/govuk_design_system_formbuilder/elements/caption.rb
index 947103c4..03edc159 100644
--- a/lib/govuk_design_system_formbuilder/elements/caption.rb
+++ b/lib/govuk_design_system_formbuilder/elements/caption.rb
@@ -3,7 +3,7 @@ module Elements
     class Caption < Base
       include Traits::Localisation
 
-      def initialize(builder, object_name, attribute_name, text: nil, size: nil, **kwargs)
+      def initialize(builder, object_name, attribute_name, text: nil, size: config.default_caption_size, **kwargs)
         super(builder, object_name, attribute_name)
 
         @text            = text(text)
@@ -28,7 +28,7 @@ def text(override)
       end
 
       def size_class(size)
-        case size || config.default_caption_size
+        case size
         when 'xl' then %(#{brand}-caption-xl)
         when 'l'  then %(#{brand}-caption-l)
         when 'm'  then %(#{brand}-caption-m)