diff --git a/edx_exams/apps/api/v1/tests/test_views.py b/edx_exams/apps/api/v1/tests/test_views.py
index 6be0f561..462dceb2 100644
--- a/edx_exams/apps/api/v1/tests/test_views.py
+++ b/edx_exams/apps/api/v1/tests/test_views.py
@@ -1961,7 +1961,7 @@ def test_post_invalid_field_value(self):
         response = self.request_api('post', self.user, self.exam.course_id, data=request_data)
         self.assertEqual(response.status_code, 400)
 
-    def test_post_invalid_missing_user(self):
+    def test_post_missing_user(self):
         """
         Test that 400 response is returned if serializer is invalid due to missing required field
         """
@@ -1971,6 +1971,16 @@ def test_post_invalid_missing_user(self):
         response = self.request_api('post', self.user, self.exam.course_id, data=request_data)
         self.assertEqual(response.status_code, 400)
 
+    def test_post_invalid_user(self):
+        """
+        Test that 400 response is returned if a username/email does not exist
+        """
+        request_data = [
+            {'exam_id': self.exam.id, 'username': 'junk', 'extra_time_mins': 45},
+        ]
+        response = self.request_api('post', self.user, self.exam.course_id, data=request_data)
+        self.assertEqual(response.status_code, 400)
+
     def test_post_unauthorized_course(self):
         """
         Test that an error is returned when atttempting to create an allowance for
diff --git a/edx_exams/apps/api/v1/views.py b/edx_exams/apps/api/v1/views.py
index 346c316c..9a07dff8 100644
--- a/edx_exams/apps/api/v1/views.py
+++ b/edx_exams/apps/api/v1/views.py
@@ -872,6 +872,11 @@ def post(self, request, course_id):
                     status=status.HTTP_400_BAD_REQUEST,
                     data={'detail': 'Exam does not exist'}
                 )
+            except User.DoesNotExist:
+                return Response(
+                    status=status.HTTP_400_BAD_REQUEST,
+                    data={'detail': 'Learner with username/email not found'}
+                )
 
             StudentAllowance.bulk_create_or_update(allowance_objects)