diff --git a/src/takopi/utils/git.py b/src/takopi/utils/git.py
index 75d1028..4279c4f 100644
--- a/src/takopi/utils/git.py
+++ b/src/takopi/utils/git.py
@@ -40,7 +40,13 @@ def git_ok(args: Sequence[str], *, cwd: Path) -> bool:
 
 
 def git_is_worktree(path: Path) -> bool:
-    return git_stdout(["rev-parse", "--is-inside-work-tree"], cwd=path) == "true"
+    top = git_stdout(
+        ["rev-parse", "--path-format=absolute", "--show-toplevel"],
+        cwd=path,
+    )
+    if not top:
+        return False
+    return Path(top).resolve(strict=False) == path.resolve(strict=False)
 
 
 def resolve_default_base(root: Path) -> str | None:
diff --git a/tests/test_worktrees.py b/tests/test_worktrees.py
index 33fcfe8..eaf9431 100644
--- a/tests/test_worktrees.py
+++ b/tests/test_worktrees.py
@@ -54,3 +54,27 @@ def test_ensure_worktree_creates_from_base(monkeypatch, tmp_path: Path) -> None:
     worktree_path = ensure_worktree(project, "feat/name")
     assert worktree_path == tmp_path / ".worktrees" / "feat" / "name"
     assert calls == [["worktree", "add", "-b", "feat/name", str(worktree_path), "main"]]
+
+
+def test_ensure_worktree_rejects_existing_non_worktree(
+    monkeypatch, tmp_path: Path
+) -> None:
+    project = ProjectConfig(
+        alias="z80",
+        path=tmp_path,
+        worktrees_dir=Path(".worktrees"),
+    )
+    worktree_path = tmp_path / ".worktrees" / "foo"
+    worktree_path.mkdir(parents=True)
+
+    def _fake_stdout(args, **kwargs):
+        if args == ["rev-parse", "--is-inside-work-tree"]:
+            return "true"
+        if args == ["rev-parse", "--path-format=absolute", "--show-toplevel"]:
+            return str(tmp_path)
+        return None
+
+    monkeypatch.setattr("takopi.utils.git.git_stdout", _fake_stdout)
+
+    with pytest.raises(WorktreeError, match="exists but is not a git worktree"):
+        ensure_worktree(project, "foo")