diff --git a/.github/workflows/doc.yml b/.github/workflows/doc.yml
index 0610569a..402c5a0f 100644
--- a/.github/workflows/doc.yml
+++ b/.github/workflows/doc.yml
@@ -4,6 +4,7 @@ on: [push, pull_request]
 
 jobs:
   build:
+    # Use Ubuntu 20.04 because doxygen 1.8.13 from Ubuntu 18.04 is broken.
     runs-on: ubuntu-20.04
 
     steps:
@@ -11,7 +12,7 @@ jobs:
 
     - name: Create Build Environment
       run: |
-        sudo apt install doxygen python-virtualenv
+        sudo apt install doxygen python3-virtualenv
         sudo npm install -g less clean-css
         cmake -E make_directory ${{runner.workspace}}/build