diff --git a/src/theme/book.js b/src/theme/book.js
index 79d40354b6..d40440c723 100644
--- a/src/theme/book.js
+++ b/src/theme/book.js
@@ -371,7 +371,14 @@ function playground_text(playground) {
     });
 
     themePopup.addEventListener('click', function (e) {
-        var theme = e.target.id || e.target.parentElement.id;
+        var theme;
+        if (e.target.className === "theme") {
+            theme = e.target.id;
+        } else if (e.target.parentElement.className === "theme") {
+            theme = e.target.parentElement.id;
+        } else {
+            return;
+        }
         set_theme(theme);
     });