diff --git a/requirements.txt b/requirements.txt
index 0e47e7a1c..18a1498a8 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,6 +5,7 @@ cymem>=1.30,<1.32
 preshed>=1.0.0,<2.0.0
 thinc>=6.10.1,<6.11.0
 murmurhash>=0.28,<0.29
+cytoolz>=0.9.0,<0.10.0
 plac<1.0.0,>=0.9.6
 ujson>=1.35
 dill>=0.2,<0.3