diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 0a32aa96a9..23a722c730 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -25,6 +25,8 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v1
+      with:
+        submodules: true
     - run: |
         set -e
         curl -L https://github.com/rust-lang-nursery/mdBook/releases/download/v0.3.1/mdbook-v0.3.1-x86_64-unknown-linux-gnu.tar.gz | tar xzf -