diff --git a/static/js/components/literate-programming.js b/static/js/components/literate-programming.js
index 4797945..7a46160 100644
--- a/static/js/components/literate-programming.js
+++ b/static/js/components/literate-programming.js
@@ -183,7 +183,9 @@ class OutputMode {
             }
         });
 
-        this.frame.placeholderImage.classList.add("loading");
+        if (this.frame.placeholderImage != null) {
+            this.frame.placeholderImage.classList.add("loading");
+        }
 
         this.frame.program.onChanged.push(_ => this.evaluate());
     }