diff --git a/mnemosyne/library/api/workspaces.py b/mnemosyne/library/api/workspaces.py
index 860cd63..3a7371b 100644
--- a/mnemosyne/library/api/workspaces.py
+++ b/mnemosyne/library/api/workspaces.py
@@ -213,14 +213,24 @@ def workspace_detail_or_delete(request, workspace_id):
     except Library.DoesNotExist:
         lib = None
 
-    # Cross-user reads look like "not found" — don't disclose existence
-    # across users. DELETE is handled separately below: telling the caller
-    # "deleted" about a library we did not touch is what silently orphans
-    # libraries, and an orphan holds its globally-unique name forever.
-    unowned = lib is not None and lib.owner_username != request.user.username
+    # A NULL owner_username is *unclaimed*, not someone else's: workspace
+    # libraries created before owner stamping (and any created through the
+    # plain /libraries/ endpoint, which never sets it) have no owner. Treating
+    # those as foreign would make them permanently undeletable through this
+    # endpoint — the very orphans this view is trying not to create.
+    #
+    # Cross-user reads still look like "not found"; DELETE is handled
+    # separately below, because telling the caller "deleted" about a library
+    # we did not touch is what silently orphans libraries, and an orphan holds
+    # its globally-unique name forever.
+    foreign = (
+        lib is not None
+        and lib.owner_username
+        and lib.owner_username != request.user.username
+    )
 
     if request.method == "GET":
-        if lib is None or unowned:
+        if lib is None or foreign:
             return Response(
                 {"detail": "Workspace not found."},
                 status=status.HTTP_404_NOT_FOUND,
@@ -231,7 +241,7 @@ def workspace_detail_or_delete(request, workspace_id):
     if lib is None:
         return Response(status=status.HTTP_204_NO_CONTENT)
 
-    if unowned:
+    if foreign:
         # Still opaque about ownership, but never a false success: the
         # caller must not record this workspace as cleaned up.
         logger.warning(
diff --git a/mnemosyne/library/tests/test_workspaces_api.py b/mnemosyne/library/tests/test_workspaces_api.py
index 402b587..04e4262 100644
--- a/mnemosyne/library/tests/test_workspaces_api.py
+++ b/mnemosyne/library/tests/test_workspaces_api.py
@@ -284,6 +284,34 @@ class WorkspaceDeleteOwnershipTests(TestCase):
         self.assertEqual(response.status_code, 204)
         cascade.assert_called_once_with(lib)
 
+    def test_unclaimed_library_is_deleted(self):
+        """A NULL owner is unclaimed, not foreign.
+
+        Workspace libraries predating owner stamping (and any created via the
+        plain /libraries/ endpoint, which never sets an owner) have
+        owner_username=None. Refusing those would make them permanently
+        undeletable through this endpoint — orphans by construction.
+        """
+        from library.models import Library
+
+        lib = self._library(None)
+        with (
+            patch("neomodel.sync_.match.NodeSet.get", return_value=lib),
+            patch(
+                "library.api.workspaces.delete_library_cascade",
+                return_value={
+                    "library_uid": "lib_1",
+                    "name": "Assistant",
+                    "item_count": 0,
+                    "orphans_deleted": 0,
+                },
+            ) as cascade,
+        ):
+            response = self.client.delete("/library/api/workspaces/ws_a/")
+
+        self.assertEqual(response.status_code, 204)
+        cascade.assert_called_once_with(lib)
+
     def test_unowned_library_get_still_looks_absent(self):
         """Ownership must stay opaque on reads — 404, not 409."""
         from library.models import Library