diff --git a/custom_requirements.txt b/custom_requirements.txt
index 9f34ef0..e52eadf 100644
--- a/custom_requirements.txt
+++ b/custom_requirements.txt
@@ -1,4 +1,4 @@
 # SPDX-License-Identifier: MIT
 
 # Write requirements for running your custom commands in tox here:
-PyYAML
+PyYAML; python_version == '2.7' or python_version >= '3.5'