diff --git a/requirements.txt b/requirements.txt
index 2f73f06..9b85250 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,7 +1,7 @@
 pip
 numpy
 matplotlib
 scipy
 plotly
-cython
+cython==0.29.15
 qutip==4.4.0
\ No newline at end of file