diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 5ef1ba628178eb22bf7f845f31796562203b5b18..250b35cab95ab77219282ea6651f37aa832ec710 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -6,7 +6,7 @@ build-kernel:
   stage: build
   script:
     - apt-get update
-    - apt-get install -y build-essential curl
+    - apt-get install -y build-essential curl flex
     - cd kernel; ./compile.bash
   tags:
   - highmem