diff --git a/.ci/Dockerfile-ubuntu-18.04 b/.ci/Dockerfile-ubuntu-18.04
index 359b758..4953f89 100644
--- a/.ci/Dockerfile-ubuntu-18.04
+++ b/.ci/Dockerfile-ubuntu-18.04
@@ -3,7 +3,7 @@ FROM ubuntu:18.04
 ARG toolchain
 
 RUN apt-get update
-RUN apt-get install -y cmake git ninja-build re2c
+RUN apt-get install -y bison cmake git ninja-build re2c
 RUN if [ "${toolchain}" = "gcc" ]; then apt-get install -y g++; fi
 RUN if [ "${toolchain}" = "clang" ]; then apt-get install -y clang; fi