diff --git a/core/collections/serializers.py b/core/collections/serializers.py
index e238d073..c7f0b038 100644
--- a/core/collections/serializers.py
+++ b/core/collections/serializers.py
@@ -199,10 +199,11 @@ def prepare_object(self, validated_data, instance=None):
         collection.full_name = validated_data.get('full_name', collection.full_name) or collection.name
         collection.autoexpand_head = validated_data.get('autoexpand_head', collection.autoexpand_head)
         collection.autoexpand = validated_data.get('autoexpand', collection.autoexpand)
-        collection.expansion_uri = validated_data.get('expansion_uri', collection.expansion_uri)
-        if collection.id and collection.expansion_uri and not collection.expansions.filter(
-                uri=collection.expansion_uri).exists():
-            self._errors.update({'expansion_url': INVALID_EXPANSION_URL})
+        # A new collection or version has no expansions yet, so only an update may name one.
+        if collection.id:
+            collection.expansion_uri = validated_data.get('expansion_uri', collection.expansion_uri)
+            if collection.expansion_uri and not collection.expansions.filter(uri=collection.expansion_uri).exists():
+                self._errors.update({'expansion_url': INVALID_EXPANSION_URL})
 
         return collection
 
diff --git a/core/integration_tests/tests_collections.py b/core/integration_tests/tests_collections.py
index 2cc5ba34..0cda6594 100644
--- a/core/integration_tests/tests_collections.py
+++ b/core/integration_tests/tests_collections.py
@@ -340,6 +340,32 @@ def test_post_201_with_autoexpand_head_false(self):
             ]
         )
 
+    def test_post_201_ignores_expansion_uri(self):
+        org = OrganizationFactory(mnemonic='org')
+        user = UserProfileFactory(organizations=[org], username='user')
+        other_expansion = ExpansionFactory(collection_version=OrganizationCollectionFactory())
+
+        for collection_id, expansion_uri, autoexpand_head in [
+                ('coll1', 'https://other.example/x/', True), ('coll2', other_expansion.uri, False)]:
+            with self.subTest(expansion_uri=expansion_uri):
+                response = self.client.post(
+                    '/orgs/org/collections/',
+                    {
+                        'id': collection_id,
+                        'name': 'Collection',
+                        'expansion_uri': expansion_uri,
+                        'autoexpand_head': autoexpand_head
+                    },
+                    HTTP_AUTHORIZATION='Token ' + user.get_token(),
+                    format='json'
+                )
+
+                self.assertEqual(response.status_code, 201)
+                collection = org.collection_set.get(mnemonic=collection_id, version='HEAD')
+                own_expansion = collection.expansions.first()
+                self.assertEqual(collection.expansion_uri, own_expansion.uri if autoexpand_head else None)
+                self.assertEqual(response.data['expansion_url'], collection.expansion_uri)
+
     def test_post_400(self):
         org = OrganizationFactory(mnemonic='org')
         user = UserProfileFactory(organizations=[org])
@@ -2429,6 +2455,25 @@ def test_put_400(self):
         self.assertEqual(response.status_code, 400)
         self.assertEqual(response.data, {'version': [ErrorDetail(string='This field may not be null.', code='null')]})
 
+    def test_put_expansion_url(self):
+        own_expansion = ExpansionFactory(collection_version=self.collection_v1)
+        other_expansion = ExpansionFactory(collection_version=self.collection)
+
+        for expansion_url, status_code in [
+                ('https://other.example/x/', 400), (other_expansion.uri, 400), (own_expansion.uri, 200)]:
+            with self.subTest(expansion_url=expansion_url):
+                response = self.client.put(
+                    self.collection_v1.uri,
+                    {'expansion_url': expansion_url},
+                    HTTP_AUTHORIZATION='Token ' + self.token,
+                    format='json'
+                )
+
+                self.assertEqual(response.status_code, status_code)
+
+        self.collection_v1.refresh_from_db()
+        self.assertEqual(self.collection_v1.expansion_uri, own_expansion.uri)
+
     @patch('core.common.models.delete_s3_objects')
     def test_delete(self, delete_s3_objects_mock):
         response = self.client.delete(
@@ -3169,6 +3214,24 @@ def test_post_201_autoexpand_false(self):
         self.assertEqual(last_created_version.expansions.count(), 0)
         self.assertEqual(last_created_version.references.count(), 1)
 
+    def test_post_201_ignores_expansion_uri(self):
+        other_expansion = ExpansionFactory(collection_version=UserCollectionFactory())
+
+        for version, expansion_uri, autoexpand in [
+                ('v1', 'https://other.example/x/', True), ('v2', other_expansion.uri, False)]:
+            with self.subTest(expansion_uri=expansion_uri):
+                response = self.client.post(
+                    self.collection.uri + 'versions/',
+                    {'id': version, 'expansion_uri': expansion_uri, 'autoexpand': autoexpand},
+                    HTTP_AUTHORIZATION='Token ' + self.token,
+                    format='json'
+                )
+
+                self.assertEqual(response.status_code, 201)
+                new_version = self.collection.versions.get(version=version)
+                own_expansion = new_version.expansions.first()
+                self.assertEqual(new_version.expansion_uri, own_expansion.uri if autoexpand else None)
+
 
 class ExportCollectionTaskTest(OCLAPITestCase):
     @patch('core.common.utils.get_export_service')