diff --git a/ci/environment.yml b/ci/environment.yml
index f71f06dbf0..e09009f980 100644
--- a/ci/environment.yml
+++ b/ci/environment.yml
@@ -15,5 +15,6 @@ dependencies:
   - xonsh
   - re2c
   - numpy
+  - zlib
 #  - bison=3.4  [not win]
 #  - m2-bison=3.4  [win]