diff --git a/openwisp_radius/api/serializers.py b/openwisp_radius/api/serializers.py
index b8f24bcb..34dafe74 100644
--- a/openwisp_radius/api/serializers.py
+++ b/openwisp_radius/api/serializers.py
@@ -151,13 +151,13 @@ class RadiusAccountingSerializer(serializers.ModelSerializer):
     framed_ipv6_address = serializers.IPAddressField(
         required=False, allow_blank=True, protocol='IPv6'
     )
-    session_time = serializers.IntegerField(required=False, default=0)
+    session_time = serializers.IntegerField(required=False)
     stop_time = serializers.DateTimeField(required=False)
     update_time = serializers.DateTimeField(required=False)
-    input_octets = serializers.IntegerField(required=False, default=0)
-    output_octets = serializers.IntegerField(required=False, default=0)
-    # this is needed otherwise serialize will ignore status_type from accounting packet
-    # as it's not a model field
+    input_octets = serializers.IntegerField(required=False)
+    output_octets = serializers.IntegerField(required=False)
+    # this is needed otherwise serializer will ignore status_type
+    # from the accounting request because it's not a model field
     status_type = serializers.ChoiceField(
         write_only=True, required=True, choices=STATUS_TYPE_CHOICES
     )
@@ -191,9 +191,24 @@ def is_valid(self, raise_exception=False):
             raise error
 
     def run_validation(self, data):
+        """
+        Custom validation to handle empty strings in:
+            - session_time
+            - input_octets
+            - output_octets
+        """
         for field in ['session_time', 'input_octets', 'output_octets']:
-            if data.get('status_type', None) == 'Start' and data[field] == '':
+            # missing data in accounting start,
+            # let's set zero as default value
+            if data.get('status_type', None) == 'Start' and data.get(field) == '':
                 data[field] = 0
+            # missing data in accounting stop,
+            # let's remove the empty string to
+            # prevent the API from failing
+            # the existing values stored in previous
+            # interim-updates won't be changed
+            if data.get('status_type', None) == 'Stop' and data.get(field) == '':
+                del data[field]
         return super().run_validation(data)
 
     def validate(self, data):
diff --git a/openwisp_radius/tests/test_api/test_freeradius_api.py b/openwisp_radius/tests/test_api/test_freeradius_api.py
index a1572651..6ad66e7c 100644
--- a/openwisp_radius/tests/test_api/test_freeradius_api.py
+++ b/openwisp_radius/tests/test_api/test_freeradius_api.py
@@ -991,6 +991,41 @@ def test_accounting_stop_200(self):
         self.assertEqual(ra.start_time, start_time)
         self.assertAcctData(ra, data)
 
+    @freeze_time(START_DATE)
+    def test_accounting_stop_empty_octets(self):
+        self.assertEqual(RadiusAccounting.objects.count(), 0)
+        data = self.acct_post_data
+        data.update(
+            dict(
+                input_octets=9900909,
+                output_octets=1513075509,
+            )
+        )
+        ra = self._create_radius_accounting(**data)
+        ra.refresh_from_db()
+        start_time = ra.start_time
+        data = self.acct_post_data
+        data.update(
+            {
+                'status_type': 'Stop',
+                'terminate_cause': '',
+                'input_octets': '',
+                'output_octets': '',
+            }
+        )
+        data = self._get_accounting_params(**data)
+        response = self.post_json(data)
+        self.assertEqual(response.status_code, 200)
+        self.assertIsNone(response.data)
+        self.assertEqual(RadiusAccounting.objects.count(), 1)
+        ra.refresh_from_db()
+        self.assertEqual(ra.update_time.timetuple(), now().timetuple())
+        self.assertEqual(ra.stop_time.timetuple(), now().timetuple())
+        self.assertEqual(ra.start_time, start_time)
+        self.assertEqual(ra.input_octets, 9900909)
+        self.assertEqual(ra.output_octets, 1513075509)
+        self.assertEqual(ra.session_time, 261)
+
     @freeze_time(START_DATE)
     @capture_any_output()
     def test_accounting_stop_201(self):