diff --git a/.ci/mitogen_install.py b/.ci/mitogen_install.py
index 9afbe67d..10813b55 100755
--- a/.ci/mitogen_install.py
+++ b/.ci/mitogen_install.py
@@ -4,7 +4,7 @@ import ci_lib
 
 batches = [
     [
-        'pip install "pycparser<2.19"',
+        'pip install "pycparser<2.19" "idna<2.7"',
         'pip install -r tests/requirements.txt',
     ],
     [