diff --git a/Dockerfile b/Dockerfile
index 32b3810..90eeeb2 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -13,7 +13,7 @@ RUN apt-get -q -y update && \
   libreadline-dev libsctp-dev libsqlite3-dev libssl-dev libtool libwebp-dev \
   libxml2-dev libxslt-dev libyaml-dev locales make make mercurial patch python3 \
   unixodbc-dev unzip wget xz-utils zlib1g-dev zsh gnupg inotify-tools less \
-  postgresql-client ssh direnv libmnl-dev bc m4 cmake cpio rsync && \
+  postgresql-client ssh direnv libmnl-dev bc m4 cmake cpio rsync flex && \
   apt-get -q -y clean
 
 RUN locale-gen en_NZ.UTF-8