diff --git a/requirements.txt b/requirements.txt
index 4dcd662..47a91a1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -7,4 +7,4 @@ tabulate>=0.8.2
 tqdm>=4.26.0
 cvxpy>=1.0.31 # for preimage.
 cvxopt>=1.2.5 # for preimage.
-mosek>=9.2.5 # for preimage.
+mosek>=9.2.5; python_version >= '3.6' # for preimage.