diff --git a/app/coffeescripts/models/QuizOverrideLoader.coffee b/app/coffeescripts/models/QuizOverrideLoader.coffee
index d697a74e2bd..25870ad8754 100644
--- a/app/coffeescripts/models/QuizOverrideLoader.coffee
+++ b/app/coffeescripts/models/QuizOverrideLoader.coffee
@@ -44,11 +44,15 @@ define [
       quiz.set('loadingOverrides', false)
 
     _chooseLatest: (dates, type) ->
+      if _.any(dates, (d) -> _.isNull(d[type]) || _.isUndefined(d[type]))
+        return null
       sortedDates = @_sortedDatesOfType(dates, type)
       if _.any(sortedDates)
         _.last(sortedDates)
 
     _chooseEarliest: (dates, type) ->
+      if _.any(dates, (d) -> _.isNull(d[type]) || _.isUndefined(d[type]))
+        return null
       sortedDates = @_sortedDatesOfType(dates, type)
       if _.any(sortedDates)
         _.first(sortedDates)
diff --git a/spec/coffeescripts/quizzes/QuizOverrideLoaderSpec.js b/spec/coffeescripts/quizzes/QuizOverrideLoaderSpec.js
index 9a6915c8697..f1a146dc80d 100644
--- a/spec/coffeescripts/quizzes/QuizOverrideLoaderSpec.js
+++ b/spec/coffeescripts/quizzes/QuizOverrideLoaderSpec.js
@@ -41,9 +41,15 @@ test('can select the earliest date from a group', function() {
   equal(this.loader._chooseEarliest(this.dates, 'unlock_at'), this.earliestDate)
 })
 
-test('ignores null dates and handles empty arrays', function() {
+test('handles null dates and handles empty arrays', function() {
   let dates = [{}, {}]
   equal(this.loader._chooseLatest(dates, 'due_at'), null)
   dates = []
   equal(this.loader._chooseLatest(dates, 'due_at'), null)
 })
+
+test('returns null if any argument is null', function() {
+  const dates = [...this.dates, {}]
+  equal(this.loader._chooseLatest(dates, 'due_at'), null)
+  equal(this.loader._chooseEarliest(dates, 'due_at'), null)
+})