diff --git a/rapids/views.py b/rapids/views.py
index f890a73e..289a871d 100755
--- a/rapids/views.py
+++ b/rapids/views.py
@@ -779,8 +779,10 @@ def mark(request, pk, sk):
     # this could be improved
     if question.normal:
         incorrect_answers = question.get_user_answers()
-        incorrect_answers.remove("")
-        incorrect_answers.remove("normal")
+        if "" in incorrect_answers:
+            incorrect_answers.remove("")
+        if "normal" in incorrect_answers:
+            incorrect_answers.remove("normal")
 
     return render(
         request,