diff --git a/devel_tools/docker/frama-c.custom/Dockerfile b/devel_tools/docker/frama-c.custom/Dockerfile
index 226b19fcf0030a9d07c9f7dfba2705e082986256..c742082bbd45bb69371125d868de56357da68e84 100644
--- a/devel_tools/docker/frama-c.custom/Dockerfile
+++ b/devel_tools/docker/frama-c.custom/Dockerfile
@@ -47,7 +47,7 @@ RUN cd /root && \
         make -j && \
         make install \
     ) && \
-    [ "${with_source}" != "no" ] || rm -rf frama-c
+    ([ "${with_source}" != "no" ] || rm -rf frama-c)
 
 # with_test: run Frama-C tests; requires "with_source=yes"
 ARG with_test=no