diff --git a/config/initializers/acsv.rb b/config/initializers/acsv.rb
index 3a24d2032..d099dbe6a 100644
--- a/config/initializers/acsv.rb
+++ b/config/initializers/acsv.rb
@@ -4,7 +4,7 @@ require 'csv'
 module ACSV
   class CSV < ::CSV
     def self.new_for_ruby3(data, options = {})
-      options[:col_sep] ||= ACSV::Detect.separator(data)
+      options[:col_sep] ||= ACSV::Detect.separator(data) || :auto
       # because of the Separation of positional and keyword arguments in Ruby 3.0
       # (https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/)
       # instead of
diff --git a/spec/controllers/administrateurs/groupe_instructeurs_controller_spec.rb b/spec/controllers/administrateurs/groupe_instructeurs_controller_spec.rb
index b3b1f1a2f..7c1693b88 100644
--- a/spec/controllers/administrateurs/groupe_instructeurs_controller_spec.rb
+++ b/spec/controllers/administrateurs/groupe_instructeurs_controller_spec.rb
@@ -405,6 +405,17 @@ describe Administrateurs::GroupeInstructeursController, type: :controller do
       it { expect(flash.alert).to eq("Import terminé. Cependant les emails suivants ne sont pas pris en compte: kara") }
     end
 
+    context 'when the csv file has only one column' do
+      let(:csv_file) { fixture_file_upload('spec/fixtures/files/valid-instructeurs-file.csv', 'text/csv') }
+
+      before { subject }
+
+      it { expect { subject }.not_to raise_error }
+      it { expect(response.status).to eq(302) }
+      it { expect(flash.alert).to be_present }
+      it { expect(flash.alert).to eq("Importation impossible, veuillez importer un csv <a href=\"/csv/#{I18n.locale}/import-groupe-test.csv\">suivant ce modèle</a>") }
+    end
+
     context 'when the file content type is application/vnd.ms-excel' do
       let(:csv_file) { fixture_file_upload('spec/fixtures/files/groupe_avec_caracteres_speciaux.csv', "application/vnd.ms-excel") }