diff --git a/app/views/green_lanes/category_assessments/index.html.erb b/app/views/green_lanes/category_assessments/index.html.erb
index 2a5e367f..214b4e67 100644
--- a/app/views/green_lanes/category_assessments/index.html.erb
+++ b/app/views/green_lanes/category_assessments/index.html.erb
@@ -21,7 +21,7 @@
           <%= form.label :regulation_role, "Regulation Role", class: "govuk-label" %>
           <%= form.text_field :regulation_role, value: params[:regulation_role], class: "govuk-input", id: "search-term"%>
           <%= form.label :theme_id, "Theme", class: "govuk-label" %>
-          <%= form.collection_select :theme_id, @themes, :id, :label, {prompt: "Select a Theme", selected: params[:theme_id]}, {class: "govuk-select govuk-!-width-one-third"}%>
+          <%= form.collection_select :theme_id, [OpenStruct.new(id: nil, label: "Select a Theme")] + @themes, :id, :label, {selected: params[:theme_id]}, {class: "govuk-select govuk-!-width-one-third"}%>
         </div>
 
         <div class="govuk-form-group">