diff --git a/course/classes/editcategory_form.php b/course/classes/editcategory_form.php
index a28ea9bf488..c866f6cee22 100644
--- a/course/classes/editcategory_form.php
+++ b/course/classes/editcategory_form.php
@@ -125,10 +125,10 @@ class core_course_editcategory_form extends moodleform {
         if (!empty($data['idnumber'])) {
             if ($existing = $DB->get_record('course_categories', array('idnumber' => $data['idnumber']))) {
                 if (!$data['id'] || $existing->id != $data['id']) {
-                    $errors['idnumber']= get_string('categoryidnumbertaken');
+                    $errors['idnumber'] = get_string('categoryidnumbertaken', 'error');
                 }
             }
         }
         return $errors;
     }
-}
\ No newline at end of file
+}
diff --git a/course/tests/behat/category_management.feature b/course/tests/behat/category_management.feature
index baed279b412..d2a4f57fced 100644
--- a/course/tests/behat/category_management.feature
+++ b/course/tests/behat/category_management.feature
@@ -332,3 +332,17 @@ Feature: Test category management actions
     And the "movecategoriesto" "select" should be disabled
     And the "resortcategoriesby" "select" should be disabled
     And the "resortcoursesby" "select" should be disabled
+
+  Scenario: Test that is not possible to create a course category with a duplicate idnumber
+    Given the following "categories" exists:
+      | name | category | idnumber |
+      | Cat 1 | 0 | CAT1 |
+    And I log in as "admin"
+    And I expand "Site administration" node
+    And I expand "Courses" node
+    And I follow "Add a category"
+    And I set the following fields to these values:
+      | Category name | Test duplicate |
+      | Category ID number | CAT1 |
+    When I press "Create category"
+    Then I should see "ID number is already used for another category"