diff --git a/app/controllers/address_search_controller.rb b/app/controllers/address_search_controller.rb
index 90d813d6d..4fc8d997a 100644
--- a/app/controllers/address_search_controller.rb
+++ b/app/controllers/address_search_controller.rb
@@ -5,7 +5,9 @@ class AddressSearchController < ApplicationController
   def index
     query = params[:query]
 
-    if query.match?(/\A\d+\z/) && query.length > 5
+    if query.nil?
+      render json: { error: "Query cannot be blank." }, status: :bad_request
+    elsif query.match?(/\A\d+\z/) && query.length > 5
       # Query is all numbers and greater than 5 digits, assume it's a UPRN
       service = UprnClient.new(query)
       service.call