diff --git a/requirements.txt b/requirements.txt
index 4b56d265cc..438be62a65 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -8,4 +8,6 @@ setuptools
 pyserial>=3.0
 future>=0.15.2
 cryptography>=2.1.4,<35
+# cffi 1.15 supports Python 3.6+ only
+cffi<1.15;python_version<'3.6'
 pyparsing>=2.0.3,<2.4.0