diff --git a/requirements.txt b/requirements.txt
index 0b465c5..3ee6b30 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 # Dev Dependencies
+cython==0.28.4
 progressbar2==3.38.0
 pycodestyle==2.4.0
 pytest==3.5.1