diff --git a/subiquity/server/controllers/filesystem.py b/subiquity/server/controllers/filesystem.py
index d6a9b318..c24c8b1a 100644
--- a/subiquity/server/controllers/filesystem.py
+++ b/subiquity/server/controllers/filesystem.py
@@ -373,10 +373,15 @@ class FilesystemController(SubiquityController, FilesystemManipulator):
         if data.partition.grub_device not in (None, partition.grub_device):
             raise ValueError('edit_partition does not support changing '
                              + 'grub_device')
-
+        existing_format = ''
+        existing_mount = ''
+        if partition._fs:
+            existing_format = partition._fs.fstype
+            if partition._fs._mount:
+                existing_mount = partition._fs._mount.path
         spec = {
-            'fstype': data.partition.format,
-            'mount': data.partition.mount,
+            'fstype': data.partition.format or existing_format,
+            'mount': data.partition.mount or existing_mount,
         }
         self.partition_disk_handler(disk, partition, spec)
         return await self.v2_GET()
diff --git a/subiquity/tests/api/test_api.py b/subiquity/tests/api/test_api.py
index c6c0e393..b4439053 100755
--- a/subiquity/tests/api/test_api.py
+++ b/subiquity/tests/api/test_api.py
@@ -475,6 +475,32 @@ class TestWin10(TestAPI):
         self.assertFalse(resp['need_root'])
         self.assertFalse(resp['need_boot'])
 
+    @timeout(5)
+    async def test_edit_partial(self):
+        disk_id = 'disk-sda'
+        await self.post('/storage/v2/reformat_disk', disk_id=disk_id)
+
+        data = {
+            'disk_id': disk_id,
+            'partition': {
+                'format': 'ext4',
+                'mount': '/',
+            }
+        }
+        await self.post('/storage/v2/add_partition', data)
+
+        data['partition'].update({
+            'number': 2,
+            'format': None,
+            'mount': '/home'
+        })
+        resp = await self.post('/storage/v2/edit_partition', data)
+
+        sda = first(resp['disks'], 'id', disk_id)
+        sda2 = first(sda['partitions'], 'number', 2)
+        self.assertEqual('ext4', sda2['format'])
+        self.assertEqual('/home', sda2['mount'])
+
 
 class TestManyDisks(TestAPI):
     machine_config = 'examples/many-nics-and-disks.json'