diff --git a/CHANGES b/CHANGES
index 7e865801719..09be46cd06f 100644
--- a/CHANGES
+++ b/CHANGES
@@ -76,6 +76,7 @@ Bugs fixed
   whitespace (ex. ``Dict[str, str]``)
 * #8945: when transforming typed fields, call the specified role instead of
   making an single xref. For C and C++, use the ``expr`` role for typed fields.
+* #9322: KeyError is raised on PropagateDescDomain transform
 
 Testing
 --------
diff --git a/sphinx/transforms/post_transforms/__init__.py b/sphinx/transforms/post_transforms/__init__.py
index dcb3da89f83..b398d077745 100644
--- a/sphinx/transforms/post_transforms/__init__.py
+++ b/sphinx/transforms/post_transforms/__init__.py
@@ -263,7 +263,8 @@ class PropagateDescDomain(SphinxPostTransform):
 
     def run(self, **kwargs: Any) -> None:
         for node in self.document.traverse(addnodes.desc_signature):
-            node['classes'].append(node.parent['domain'])
+            if node.parent.get('domain'):
+                node['classes'].append(node.parent['domain'])
 
 
 def setup(app: Sphinx) -> Dict[str, Any]: