diff --git a/edit_coderunner_form.php b/edit_coderunner_form.php
index 50f04f940..ccbdc5458 100644
--- a/edit_coderunner_form.php
+++ b/edit_coderunner_form.php
@@ -904,6 +904,11 @@ public function validation($data, $files) {
             $errors = array_merge($errors, $testcaseerrors);
         }
 
+        if ($data['prototypetype'] == 2 && empty($data['language'])) {
+            // Language cannot be empty when it is a prototype template.
+            $errors['languages'] = get_string('emptysandboxlanguage', 'qtype_coderunner');
+        }
+
         if ($data['iscombinatortemplate'] && empty($data['testsplitterre'])) {
             $errors['templatecontrols'] = get_string('bad_empty_splitter', 'qtype_coderunner');
         }
diff --git a/lang/en/qtype_coderunner.php b/lang/en/qtype_coderunner.php
index 6fc357286..c68ddb499 100644
--- a/lang/en/qtype_coderunner.php
+++ b/lang/en/qtype_coderunner.php
@@ -146,6 +146,7 @@
 $string['editingcoderunner'] = 'Editing a CodeRunner Question';
 $string['empty_new_prototype_name'] = 'New question type name cannot be empty';
 $string['emptypenaltyregime'] = 'Penalty regime must be defined (since version 3.1)';
+$string['emptysandboxlanguage'] = 'Sandbox language cannot be empty when creating a prototype.';
 $string['enable'] = 'Enable';
 $string['enablecombinator'] = 'Enable combinator';
 $string['enable_diff_check'] = 'Enable \'Show differences\' button';
diff --git a/tests/behat/make_prototype.feature b/tests/behat/make_prototype.feature
index c083b6d57..b1b1617f9 100644
--- a/tests/behat/make_prototype.feature
+++ b/tests/behat/make_prototype.feature
@@ -77,3 +77,13 @@ Feature: make_prototype
     And I set the field "prototypetype" to "No"
     And I set the field "id_coderunnertype" to "python3" and dismiss the alert
     Then I should not see "This is a prototype; cannot change question type"
+
+  Scenario: As a teacher, when I try to create the prototype with empty Sandbox language I should see the validation error
+    Given I am on the "PROTOTYPE_test_prototype" "core_question > edit" page logged in as teacher1
+    And I click on "a[aria-controls='id_advancedcustomisationheadercontainer']" "css_element"
+    When I set the field "language" to ""
+    And I press "id_submitbutton"
+    Then I should see "Sandbox language cannot be empty when creating a prototype."
+    And I set the field "language" to "python3"
+    And I press "id_submitbutton"
+    And I should see "Question bank"