diff --git a/app/service/statistics.py b/app/service/statistics.py
index ac148795b..9b76f4d0f 100644
--- a/app/service/statistics.py
+++ b/app/service/statistics.py
@@ -92,7 +92,7 @@ def create_empty_monthly_notification_status_stats_dict(year):
     # nested dicts - data[month][template type][status] = count
     return {
         convert_utc_to_bst(start).strftime('%Y-%m'): {
-            template_type: {}
+            template_type: defaultdict(int)
             for template_type in TEMPLATE_TYPES
         }
         for start in utc_month_starts
@@ -103,6 +103,6 @@ def add_monthly_notification_status_stats(data, stats):
     for row in stats:
         month = row.month.strftime('%Y-%m')
 
-        data[month][row.notification_type][row.notification_status] = row.count
+        data[month][row.notification_type][row.notification_status] += row.count
 
     return data
diff --git a/tests/app/service/test_statistics.py b/tests/app/service/test_statistics.py
index a1aea04d4..d002ea4bc 100644
--- a/tests/app/service/test_statistics.py
+++ b/tests/app/service/test_statistics.py
@@ -194,13 +194,13 @@ def test_add_monthly_notification_status_stats():
     # this data won't be affected
     data['2018-05']['email']['sending'] = 32
 
-    # this data will get overwritten
+    # this data will get combined with the 8 from row_data
     data['2018-05']['sms']['sending'] = 16
 
     add_monthly_notification_status_stats(data, rows)
 
     assert data == {
         '2018-04': {'sms': {'sending': 1, 'delivered': 2}, 'email': {'sending': 4}, 'letter': {}},
-        '2018-05': {'sms': {'sending': 8}, 'email': {'sending': 32}, 'letter': {}},
+        '2018-05': {'sms': {'sending': 24}, 'email': {'sending': 32}, 'letter': {}},
         '2018-06': {'sms': {}, 'email': {}, 'letter': {}},
     }