diff --git a/docs/format-changelog.rst b/docs/format-changelog.rst
index f16d22c2..5e3504a0 100644
--- a/docs/format-changelog.rst
+++ b/docs/format-changelog.rst
@@ -171,3 +171,13 @@ Added
 ~~~~~
 
 - The key ``artifacts`` is introduced to describe expected build artifacts.
+
+Version 18
+----------
+
+Added
+~~~~~
+
+The repo keys ``tag`` and ``branch`` can now be set to ``null`` to remove
+these properties from the repo. This is needed in case a default value is
+set in the ``defaults`` section that should not apply to a repo.
diff --git a/docs/userguide/project-configuration.rst b/docs/userguide/project-configuration.rst
index a0f51e38..536eefbd 100644
--- a/docs/userguide/project-configuration.rst
+++ b/docs/userguide/project-configuration.rst
@@ -346,15 +346,17 @@ Configuration reference
       ``commit``, ``branch`` or ``tag``, the revision you get depends on the
       defaults of the version control system used.
 
-    ``branch``: string [optional]
+    ``branch``: string or nothing (``null``) [optional]
       The upstream branch that should be tracked. If ``commit`` was
       specified, kas checks that the branch contains the commit. If no
       ``commit`` was specified, the head of the upstream branch is checked out.
+      The nothing (``null``) value is used to remove a possible default value.
 
-    ``tag``: string [optional]
+    ``tag``: string or nothing (``null``)  [optional]
       The tag that should be checked out. If a ``commit`` was specified, kas
       checks that the tag points to this commit. This must not be combined
-      with ``branch``.
+      with ``branch``. The nothing (``null``) value is used to remove a
+      possible default value.
 
     ``path``: string [optional]
       The path where the repository is stored.
diff --git a/kas/__version__.py b/kas/__version__.py
index 9ffb4078..874a72b8 100644
--- a/kas/__version__.py
+++ b/kas/__version__.py
@@ -28,5 +28,5 @@
 __version__ = '4.5'
 
 # Please update docs/format-changelog.rst when changing the file version.
-__file_version__ = 17
+__file_version__ = 18
 __compatible_file_version__ = 1
diff --git a/kas/schema-kas.json b/kas/schema-kas.json
index 7c1e4fc5..fb267594 100644
--- a/kas/schema-kas.json
+++ b/kas/schema-kas.json
@@ -170,10 +170,24 @@
                                 "type": "string"
                             },
                             "branch": {
-                                "type": "string"
+                                "oneOf": [
+                                    {
+                                        "type": "string"
+                                    },
+                                    {
+                                        "type": "null"
+                                    }
+                                ]
                             },
                             "tag": {
-                                "type": "string"
+                                "oneOf": [
+                                    {
+                                        "type": "string"
+                                    },
+                                    {
+                                        "type": "null"
+                                    }
+                                ]
                             },
                             "refspec": {
                                 "type": "string"