diff --git a/requirements.txt b/requirements.txt
index 1bedf4e..69498d0 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,5 +1,6 @@
 -e git://github.com/mvcisback/hypothesis-cfg@master#egg=hypothesis-cfg
 -e git://github.com/mvcisback/DiscreteSignals@master#egg=discrete-signals
+attrs==18.1.0
 funcy==1.9.1
 lenses==0.4.0
 parsimonious==0.7.0