diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 32dc0799..d6e64d34 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -1,8 +1,7 @@
 # from https://stackoverflow.com/questions/51866926
 # and https://github.com/jangorecki/r.gitlab.ci
 
-# https://hub.docker.com/r/rocker/
-# image: rocker/r-base
+# how the images work: https://hub.docker.com/r/rocker/r-ver/
 
 stages:
     - build
@@ -14,7 +13,7 @@ R 3.3:
     script:
     - apt-get update
     # install dependencies for package
-    - apt-get install --yes --no-install-recommends libxml2 libssl-dev libcurl4-openssl-dev
+    - apt-get install --yes --no-install-recommends libxml2 libssl-dev libcurl4-openssl-dev zlib1g-dev
     - R -e 'install.packages(c("devtools", "testthat"))'
     - R -e 'devtools::install_deps(dependencies = c("Depends", "Imports", "Suggests"))'
     # remove vignettes folder and get VignetteBuilder field out of DESCRIPTION file
@@ -30,7 +29,7 @@ R 3.4:
     script:
     - apt-get update
     # install dependencies for package
-    - apt-get install --yes --no-install-recommends libxml2 libssl-dev libcurl4-openssl-dev
+    - apt-get install --yes --no-install-recommends libxml2 libssl-dev libcurl4-openssl-dev zlib1g-dev
     - R -e 'install.packages(c("devtools", "testthat"))'
     - R -e 'devtools::install_deps(dependencies = c("Depends", "Imports", "Suggests"))'
     # remove vignettes folder and get VignetteBuilder field out of DESCRIPTION file
@@ -46,7 +45,7 @@ R 3.5:
     script:
     - apt-get update
     # install dependencies for package
-    - apt-get install --yes --no-install-recommends libxml2 libssl-dev libcurl4-openssl-dev
+    - apt-get install --yes --no-install-recommends libxml2 libssl-dev libcurl4-openssl-dev zlib1g-dev
     - R -e 'install.packages(c("devtools", "testthat"))'
     - R -e 'devtools::install_deps(dependencies = c("Depends", "Imports", "Suggests"))'
     # remove vignettes folder and get VignetteBuilder field out of DESCRIPTION file