diff --git a/ci/deploy-book.sh b/ci/deploy-book.sh
index 0c2a2b5ff9..ba38697ed5 100755
--- a/ci/deploy-book.sh
+++ b/ci/deploy-book.sh
@@ -4,7 +4,7 @@ set -xeu
 cd "$(dirname "$0")/../book"
 
 # Ensure mdbook is installed.
-cargo install mdbook || true
+cargo install mdbook --vers "^0.0.22" || true
 export PATH="$PATH:~/.cargo/bin"
 
 # Get the git revision we are on.
diff --git a/ci/test-book.sh b/ci/test-book.sh
index 30b233180f..b2007e7c59 100755
--- a/ci/test-book.sh
+++ b/ci/test-book.sh
@@ -3,7 +3,7 @@
 set -xeu
 cd "$(dirname "$0")/../book"
 
-cargo install mdbook || true
+cargo install mdbook --vers "^0.0.22" || true
 export PATH="$PATH:~/.cargo/bin"
 
 mdbook build