diff --git a/db/migrate/20250522201031_stronger_unique_index_on_security.rb b/db/migrate/20250522201031_stronger_unique_index_on_security.rb
new file mode 100644
index 00000000..43672f81
--- /dev/null
+++ b/db/migrate/20250522201031_stronger_unique_index_on_security.rb
@@ -0,0 +1,13 @@
+class StrongerUniqueIndexOnSecurity < ActiveRecord::Migration[7.2]
+  def change
+    remove_index :securities, [ :ticker, :exchange_operating_mic ], unique: true
+
+    # Matches our ActiveRecord validation:
+    # - uppercase ticker
+    # - either exchange_operating_mic or empty string (unique index doesn't work with NULL values)
+    add_index :securities,
+              "UPPER(ticker), COALESCE(UPPER(exchange_operating_mic), '')",
+              unique: true,
+              name: "index_securities_on_ticker_and_exchange_operating_mic_unique"
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 9f079956..11612da4 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.2].define(version: 2025_05_21_112347) do
+ActiveRecord::Schema[7.2].define(version: 2025_05_22_201031) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pgcrypto"
   enable_extension "plpgsql"
@@ -517,9 +517,9 @@ ActiveRecord::Schema[7.2].define(version: 2025_05_21_112347) do
     t.datetime "failed_fetch_at"
     t.integer "failed_fetch_count", default: 0, null: false
     t.datetime "last_health_check_at"
+    t.index "upper((ticker)::text), COALESCE(upper((exchange_operating_mic)::text), ''::text)", name: "index_securities_on_ticker_and_exchange_operating_mic_unique", unique: true
     t.index ["country_code"], name: "index_securities_on_country_code"
     t.index ["exchange_operating_mic"], name: "index_securities_on_exchange_operating_mic"
-    t.index ["ticker", "exchange_operating_mic"], name: "index_securities_on_ticker_and_exchange_operating_mic", unique: true
   end
 
   create_table "security_prices", id: :uuid, default: -> { "gen_random_uuid()" }, force: :cascade do |t|