diff --git a/question.php b/question.php
index 0b88a0c..4061e27 100755
--- a/question.php
+++ b/question.php
@@ -188,7 +188,7 @@ public function grade_response(array $response) : array {
      * @param string $markscheme
      * @return string;
      */
-    public function build_full_ai_prompt($response, $aiprompt, $defaultmark, $markscheme) {
+    public function build_full_ai_prompt($response, $aiprompt, $defaultmark, $markscheme): string {
         $responsetext = strip_tags($response);
             $responsetext = '[['.$responsetext.']]';
             $prompt = get_config('qtype_aitext', 'prompt');