diff --git a/setup.cfg b/setup.cfg
index 2d681bfe6b..3e43450fa4 100644
--- a/setup.cfg
+++ b/setup.cfg
@@ -55,6 +55,7 @@ install_requires =
     sphinx>=4.4
     sphinx_rtd_theme>=0.5.0
     sphinxcontrib-svg2pdfconverter
+    markupsafe<2.1  # remove me once patched in cylc-flow
 
 [options.packages.find]
 include = cylc*