diff --git a/Dockerfile b/Dockerfile
index f4bb854..b46017e 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -36,7 +36,7 @@ RUN for l in `cat /tmp/r_non_cran_pkgs.txt` ; do \
 
 # Install TMB from sources
 RUN pkg='TMB' ; \
-    mirror=`https://cloud.r-project.org/` ; \
+    mirror='https://cloud.r-project.org' ; \
     Rscript --slave --no-site-file --no-init-file --no-save --no-restore-history \
     -e "install.packages(pkgs='$pkg', repos=c('$mirror/'));"