diff --git a/.circleci/config.yml b/.circleci/config.yml
index 1f1cb94..fd1b722 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -37,7 +37,7 @@ env: &env
       # version.
       #STACKVER: "1.6.5"
       #STACK_UPGRADE: "y"
-      #RESOLVER: "lts-12"
+      #RESOLVER: "lts-21"
       STACK_YAML: "stack.yaml"
 
       # ------------------------------------------------------------------------
@@ -88,6 +88,7 @@ preinstall: &preinstall
 
         # required for outbound https for stack and for stack setup
         apt-get install -y netbase xz-utils make
+        apt-get install -y zlib1g-dev
 
         # For ghcup to install ghc
         if test -n "$GHCUPVER"