diff --git a/problemtools/verifyproblem.py b/problemtools/verifyproblem.py
index 7d052e25..20be508b 100644
--- a/problemtools/verifyproblem.py
+++ b/problemtools/verifyproblem.py
@@ -986,6 +986,8 @@ def check(self, args):
         return self._check_res
 
     def grade(self, sub_results, testcasegroup, shadow_result=False):
+        if not sub_results:
+            return ("?", 0.0)
 
         if testcasegroup.config['grading'] == 'default':
             graders = [self._default_grader]