diff --git a/rowan/project.py b/rowan/project.py
index 30e1589..da5d842 100644
--- a/rowan/project.py
+++ b/rowan/project.py
@@ -197,8 +197,15 @@ def default_project() -> Project:
 
     Raises:
         httpx.HTTPStatusError: API request fails
+        ValueError: the user has no default project in the API key's organization
     """
     with api_client() as client:
         response = client.get("/user/me/default_project")
         response.raise_for_status()
-        return Project(**response.json())
+        project = response.json()
+    if project is None:
+        raise ValueError(
+            "You have no default project in this organization. "
+            "Set a default project, or pass a folder or project."
+        )
+    return Project(**project)
diff --git a/tests/test_project.py b/tests/test_project.py
new file mode 100644
index 0000000..a06f4cf
--- /dev/null
+++ b/tests/test_project.py
@@ -0,0 +1,35 @@
+"""Tests for project retrieval."""
+
+from collections.abc import Iterator
+from contextlib import contextmanager
+from unittest.mock import MagicMock
+
+import pytest
+
+from rowan import project as project_module
+
+
+def _mock_client(monkeypatch: pytest.MonkeyPatch, payload: dict | None) -> None:
+    client = MagicMock()
+    client.get.return_value.json.return_value = payload
+
+    @contextmanager
+    def mock_api_client() -> Iterator[MagicMock]:
+        yield client
+
+    monkeypatch.setattr(project_module, "api_client", mock_api_client)
+
+
+def test_default_project_returns_the_project(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Return the default project the API reports."""
+    _mock_client(monkeypatch, {"uuid": "project-uuid", "name": "Default"})
+
+    assert project_module.default_project().uuid == "project-uuid"
+
+
+def test_default_project_without_one_raises_a_clear_error(monkeypatch: pytest.MonkeyPatch) -> None:
+    """Explain a missing default project instead of failing to build one from null."""
+    _mock_client(monkeypatch, None)
+
+    with pytest.raises(ValueError, match="no default project"):
+        project_module.default_project()