diff --git a/src/canvaslms/grades/disjunctmax.nw b/src/canvaslms/grades/disjunctmax.nw
index 8e10929..4fb6e95 100644
--- a/src/canvaslms/grades/disjunctmax.nw
+++ b/src/canvaslms/grades/disjunctmax.nw
@@ -88,7 +88,7 @@ def summarize(user, assignments_list):
       dates.append(grade_date)
 
   if grades:
-    final_grade = grade_max(grades)
+    final_grade = grade_max(grades) or "F"
   else:
     final_grade = "F"
 
@@ -109,16 +109,23 @@ We also include P/F here, since we can count them as lower than A--E.
 <<helper functions>>=
 def grade_max(grades):
   """Takes a list of A--E/P--F grades, returns the maximum."""
-  num_grades = map(grade_to_int, grades)
-  max_grade = max(num_grades)
-  return int_to_grade(max_grade)
+  num_grades = list(map(grade_to_int,
+                        filter(lambda x: x[0] != "F", grades)))
+  if not num_grades:
+    max_grade = max(num_grades)
+    return int_to_grade(max_grade)
+  return None
 
 def grade_to_int(grade):
-  grade_map = {"F": -1, "P": 0, "E": 1, "D": 2, "C": 3, "B": 4, "A": 5}
+  grade_map = {"F": -2, "Fx": -1,
+               "P": 0,
+               "E": 1, "D": 2, "C": 3, "B": 4, "A": 5}
   return grade_map[grade]
 
 def int_to_grade(int_grade):
-  grade_map_inv = {-1: "F", 0: "P", 1: "E", 2: "D", 3: "C", 4: "B", 5: "A"}
+  grade_map_inv = {-2: "F", -1: "Fx",
+                   0: "P",
+                   1: "E", 2: "D", 3: "C", 4: "B", 5: "A"}
   return grade_map_inv[int_grade]
 @
 
diff --git a/src/canvaslms/grades/maxgradesurvey.nw b/src/canvaslms/grades/maxgradesurvey.nw
index 029a105..692c440 100644
--- a/src/canvaslms/grades/maxgradesurvey.nw
+++ b/src/canvaslms/grades/maxgradesurvey.nw
@@ -67,7 +67,7 @@ def summarize(user, assignments_list):
   for assignment in assignments_list:
     try:
       submission = assignment.get_submission(user,
-                                            include=["submission_history"])
+                                             include=["submission_history"])
     except ResourceDoesNotExist:
       grades.append("F")
       continue
@@ -91,7 +91,7 @@ def summarize(user, assignments_list):
   if len(dates) < len(grades):
     final_grade = "F"
   else:
-    final_grade = grade_max(grades)
+    final_grade = grade_max(grades) or "F"
 
   if dates:
     final_date = max(dates)