diff --git a/questions.py b/questions.py
index 0386a1c..06d60a6 100644
--- a/questions.py
+++ b/questions.py
@@ -266,16 +266,21 @@ class QuestionCheckbox(Question):
             self['grade'] = 0.0
         else:
             # answered
-            x = 0.0
+            sum_abs = sum(abs(p) for p in self['correct'])
+            if sum_abs < 1e-6:  # in case correct: [0,0,0,0,0]
+                self['grade'] = 0.0
 
-            if self['discount']:
-                for i, p in enumerate(self['correct']):
-                    x += p if str(i) in self['answer'] else -p
-                self['grade'] = x / sum(abs(p) for p in self['correct'])
             else:
-                for i, p in enumerate(self['correct']):
-                    x += p if str(i) in self['answer'] else 0.0
-                self['grade'] = x / sum(abs(p) for p in self['correct'])
+                x = 0.0
+
+                if self['discount']:
+                    for i, p in enumerate(self['correct']):
+                        x += p if str(i) in self['answer'] else -p
+                else:
+                    for i, p in enumerate(self['correct']):
+                        x += p if str(i) in self['answer'] else 0.0
+
+                self['grade'] = x / sum_abs
 
         return self['grade']
 
--
libgit2 0.21.2