diff --git a/.circleci/config.yml b/.circleci/config.yml
index d9fb01d..1437782 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -39,6 +39,7 @@ jobs:
       - run:
           name: Install pyproj
           command: |
+            # requires libstdc++6 >= 9
             sudo apt-get install -y libproj-dev proj-bin proj-data libc6 libgeos-dev python-gdal gdal-bin
             export PYPROJMAJVER=15
             export PYPROJVERSION=6.3.1-1