diff --git a/requirements.txt b/requirements.txt
index aee5ad7..1bcbf71 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,3 +5,7 @@
 #libsass==0.7.0 # G++ >= 4.8 needed to compile and NA on Debian wheezy
 pyScss==1.3.4 # API may change
 slimit==0.8.1
+
+# Ply 3.6 has an issue that causes slimit to fail
+# https://github.com/dabeaz/ply/issues/63
+ply==3.4