diff --git a/.buildbot.sh b/.buildbot.sh
index 2db6178..04a568b 100644
--- a/.buildbot.sh
+++ b/.buildbot.sh
@@ -5,10 +5,11 @@ set -e
 export CARGO_HOME="`pwd`/.cargo"
 export RUSTUP_HOME="`pwd`/.rustup"
 
+# Install no toolchain initially to ensure toolchain rollback.
 curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs > rustup.sh
-sh rustup.sh --default-host x86_64-unknown-linux-gnu --default-toolchain nightly -y --no-modify-path
-
+sh rustup.sh --default-host x86_64-unknown-linux-gnu --default-toolchain none -y --no-modify-path
 export PATH=`pwd`/.cargo/bin/:$PATH
+rustup install nightly
 
 cargo fmt --all -- --check