[Python] UV package manager cannot find my workspace member

I intended to use git submodule to bring my other source code(backend) into my-app. Then I wanted utilize uv's workspace so that it manages the dependencies of both my-app and the backend altogether.

Tree structure (the root directory name is also my-app it's denoted by . here):

.
├── my-app
│ ├── __init__.py
│ ├── __pycache__
│ ├── api
│ ├── clients
|
├── justfile
├── model.py
├── packages
│ └── backend ###### here
├── pyproject.toml
├── README.md
├── tests
│ ├── __pycache__
│ ├── conftest.py
├── uv.lock


my-app's pyproject.toml:

[project]
name = "my-app"
version = "0.1.0"
description = "Add your description here"
readme = "README.md"
requires-python = "==3.11.4"
dependencies = [
...
"rich>=13.8.1",
"backend" ##### here
]

[tool.uv.sources]
backend = { workspace = true }

[tool.uv.workspace]
members = ["packages/*"]

[build-system]
requires = ["uv_build>=0.8.7,<0.9.0"]
build-backend = "uv_build"

# other configs


And here is the error message:

error: Failed to generate package metadata for `my-app==0.1.0 @ editable+.`
Caused by: Failed to parse entry: `backend`
Caused by: `backend` references a workspace in `tool.uv.sources` (e.g., `backend = { workspace = true }`), but is not a workspace member


with those two blocks in toml file I specified that backend should be a member. In fact the example in the documentation did the same thing. The location of the directories are also the same between my directories and the docs example.

let me know if I have to provide more information

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