diff --git a/tests/docker/dockerfiles/debian9.docker b/tests/docker/dockerfiles/debian9.docker
index 056e5389cc..a4509950e6 100644
--- a/tests/docker/dockerfiles/debian9.docker
+++ b/tests/docker/dockerfiles/debian9.docker
@@ -20,6 +20,7 @@ RUN DEBIAN_FRONTEND=noninteractive eatmydata \
         bison \
         build-essential \
         ca-certificates \
+        clang \
         flex \
         git \
         pkg-config \