diff --git a/pkgs/top-level/python-packages.nix b/pkgs/top-level/python-packages.nix
index c35b3f825dd..ec14f43c008 100644
--- a/pkgs/top-level/python-packages.nix
+++ b/pkgs/top-level/python-packages.nix
@@ -6311,7 +6311,9 @@ let
     # https://github.com/paramiko/paramiko/issues/449
     doCheck = !(isPyPy || isPy33);
     checkPhase = ''
+      # test_util needs to resolve an hostname, thus failing when the fw blocks it
       sed '/UtilTest/d' -i test.py
+
       ${python}/bin/${python.executable} test.py --no-sftp --no-big-file
     '';