diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 0c608db..e5cafe2 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -28,7 +28,7 @@ jobs:
         docs-folder: "docs/"
 
     # Building as a epub.
-    - uses: ammaraskar/sphinx-action@master
+    - uses: ammaraskar/sphinx-action@8.1.3
       with:
         build-command: "make epub"
         docs-folder: "docs/"
diff --git a/.github/workflows/pull_requests.yml b/.github/workflows/pull_requests.yml
index 4bead1b..78248a4 100644
--- a/.github/workflows/pull_requests.yml
+++ b/.github/workflows/pull_requests.yml
@@ -10,6 +10,6 @@ jobs:
     steps:
     - uses: actions/checkout@v1
     # Standard drop-in approach that should work for most people.
-    - uses: ammaraskar/sphinx-action@master
+    - uses: ammaraskar/sphinx-action@8.1.3
       with:
         docs-folder: "docs/"