diff --git a/app/controllers/dmsf_links_controller.rb b/app/controllers/dmsf_links_controller.rb
index e591e6f6..4c60d77c 100644
--- a/app/controllers/dmsf_links_controller.rb
+++ b/app/controllers/dmsf_links_controller.rb
@@ -121,9 +121,9 @@ class DmsfLinksController < ApplicationController
       end
     else
       # Link to
-      @dmsf_link.dmsf_folder_id = if DmsfLinksHelper.number?(params[:dmsf_link][:target_folder_id])
-                                    params[:dmsf_link][:target_folder_id].to_i
-                                  end
+      if DmsfLinksHelper.number?(params[:dmsf_link][:target_folder_id])
+        @dmsf_link.dmsf_folder_id = params[:dmsf_link][:target_folder_id].to_i
+      end
       if params[:dmsf_link][:target_project_id].present?
         @dmsf_link.project_id = params[:dmsf_link][:target_project_id]
       else
diff --git a/app/helpers/dmsf_links_helper.rb b/app/helpers/dmsf_links_helper.rb
index a149feee..67ba38d0 100644
--- a/app/helpers/dmsf_links_helper.rb
+++ b/app/helpers/dmsf_links_helper.rb
@@ -32,12 +32,12 @@ module DmsfLinksHelper
 
   # An integer test
   def self.number?(str)
-    str.to_s.match(/\A[+-]?\d+?(\.\d+)?\Z/).nil?
+    str&.match?(/\A\d+\Z/)
   end
 
   def files_for_select(project_id, folder_id)
     files = []
-    if folder_id && DmsfLinksHelper.number?(folder_id)
+    if DmsfLinksHelper.number?(folder_id)
       folder = DmsfFolder.find_by(id: folder_id)
       files = folder.dmsf_files.visible.to_a if folder
     elsif project_id
diff --git a/test/helpers/dmsf_links_helper_test.rb b/test/helpers/dmsf_links_helper_test.rb
new file mode 100644
index 00000000..78ab3195
--- /dev/null
+++ b/test/helpers/dmsf_links_helper_test.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+#
+# Redmine plugin for Document Management System "Features"
+#
+# Copyright © 2011-23 Karel Pičman <karel.picman@kontron.com>
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License
+# as published by the Free Software Foundation; either version 2
+# of the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+
+require File.expand_path('../../test_helper', __FILE__)
+
+# Links helper
+class DmsfLinksHelperTest < RedmineDmsf::Test::HelperTest
+  include DmsfLinksHelper
+
+  def test_number
+    assert DmsfLinksHelper.number?('123')
+    assert_not DmsfLinksHelper.number?('-123')
+    assert_not DmsfLinksHelper.number?('12.3')
+    assert_not DmsfLinksHelper.number?('123a')
+    assert_not DmsfLinksHelper.number?(nil)
+  end
+end