diff --git a/.circleci/config.yml b/.circleci/config.yml
index 7089f77..f51d85d 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -36,6 +36,7 @@ references:
       command: |
         python3 -m venv venv
         venv/bin/pip install -U pip setuptools wheel
+        venv/bin/pip install -U 'Sphinx <7'
         venv/bin/pip install -r all-requirements.txt
   save-dependencies-cache: &save-dependencies-cache
     save_cache:
diff --git a/setup.py b/setup.py
index 581a86c..dd72273 100755
--- a/setup.py
+++ b/setup.py
@@ -53,6 +53,8 @@ def run(self):
         'xlwt-future >=0.8, <1',
     ],
     setup_requires=[
+        # Note: Sphinx needs to be manually installed before running `setup.py`
+        # to avoid issues importing `sphinxcontrib.applehelp`.
         'Sphinx >=2, <7',
         'Pygments >=2.0, <3',
     ],