diff --git a/app/models/champs/commune_champ.rb b/app/models/champs/commune_champ.rb
index 67f448e4b..a1e69ba48 100644
--- a/app/models/champs/commune_champ.rb
+++ b/app/models/champs/commune_champ.rb
@@ -4,6 +4,9 @@ class Champs::CommuneChamp < Champs::TextChamp
   store_accessor :value_json, :code_departement, :code_postal, :code_region
   before_save :on_codes_change, if: :should_refresh_after_code_change?
 
+  validates :external_id, presence: true, if: -> { validate_champ_value_or_prefill? && value.present? }
+  after_validation :instrument_external_id_error, if: -> { errors.include?(:external_id) }
+
   def departement_name
     APIGeoService.departement_name(code_departement)
   end
@@ -101,4 +104,11 @@ class Champs::CommuneChamp < Champs::TextChamp
   def should_refresh_after_code_change?
     !departement? || code_postal_changed? || external_id_changed?
   end
+
+  def instrument_external_id_error
+    Sentry.capture_message(
+      "Commune with value and no external id Edge case reached",
+      extra: { request_id: Current.request_id }
+    )
+  end
 end
diff --git a/spec/models/champs/commune_champ_spec.rb b/spec/models/champs/commune_champ_spec.rb
index 5448aa048..97e5e8569 100644
--- a/spec/models/champs/commune_champ_spec.rb
+++ b/spec/models/champs/commune_champ_spec.rb
@@ -28,6 +28,22 @@ describe Champs::CommuneChamp do
       expect(champ.for_export(:departement)).to eq '63 – Puy-de-Dôme'
     end
 
+    context 'with tricky bug (should not happen, but it happens)' do
+      let(:champ) do
+        described_class.new(stable_id: 99, dossier:).tap do |champ|
+          champ.external_id = ''
+          champ.value = 'Gagny'
+          champ.run_callbacks(:save)
+        end
+      end
+
+      it 'fails' do
+        expect(champ).to receive(:instrument_external_id_error)
+        expect(champ.validate(:champs_public_value)).to be_falsey
+        expect(champ.errors).to include('external_id')
+      end
+    end
+
     context 'with code' do
       let(:champ) do
         described_class.new(stable_id: 99, dossier:).tap do |champ|