diff --git a/requirements.txt b/requirements.txt
index b786b42..a68620f 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -9,6 +9,7 @@
 setuptools<50.0.0  # https://github.com/pypa/setuptools/commit/04e3df22df840c6bb244e9b27bc56750c44b7c85
 
 # Build requirements
+cffi==1.14.6; python_version < '3.6'  # cffi 1.15.0 drops support for py35.
 charm-tools==2.8.3
 
 simplejson