diff --git a/quiz.js b/quiz.js
index 1820b81..a916a2f 100644
--- a/quiz.js
+++ b/quiz.js
@@ -69,7 +69,9 @@ function shuffle(a) {
 function shuffle_cards(section) {
     let a = [];
     for (let i=0; i<cards_content[section].length; i++) {
-        a.push(i);
+        if (cards_content[section][i] != null) {
+            a.push(i);
+        }
     }
     shuffle(a);
     return a;