diff --git a/Makefile b/Makefile
@@ -58,6 +58,7 @@ venv:
 .PHONY: ensure-venv
 ensure-venv:
 	@if [ ! -d $(VENVDIR) ] ; then \
+		set -e; \
 		echo "Creating venv in $(VENVDIR)"; \
 		if $(UV) --version >/dev/null 2>&1; then \
 			$(UV) venv --python=$(PYTHON) $(VENVDIR); \

diff --git a/documentation/start-documenting.rst b/documentation/start-documenting.rst
@@ -76,12 +76,22 @@ To build the documentation, follow the steps in one of the sections below.
 You can view the documentation after building the HTML
 by opening the file :file:`Doc/build/html/index.html` in a web browser.
 
-.. note::
+Initial requirements
+--------------------
+
+Ensure your current working directory is the top level ``Doc/`` directory
+inside your :ref:`CPython repository clone <checkout>`. You can switch to
+it with:
+
+.. code-block:: shell
+
+   cd Doc
+
+Ensure your Python version is at least 3.11. You can verify it with:
 
-   The following instructions all assume your current working dir is
-   the ``Doc`` subdirectory in your :ref:`CPython repository clone <checkout>`.
-   Make sure to switch to it with ``cd Doc`` if necessary.
+.. code-block:: shell
 
+   python --version
 
 .. _doc-create-venv: