diff --git a/tox.ini b/tox.ini
index 927070a9c4..0d95801e13 100644
--- a/tox.ini
+++ b/tox.ini
@@ -109,7 +109,7 @@ commands =
     # Make all greater-thans equals so we test the oldest version of our direct
     # dependencies, but make the pyopenssl 17.0, which can work against an
     # OpenSSL 1.1 compiled cryptography (as older ones don't compile on Travis).
-    sed -ibackup -e "s/>=/==/g" -e "/psycopg2/d" -e "s/pyopenssl==16.0.0/pyopenssl==17.0.0/" pyproject.toml
+    /bin/sh 'sed -ibackup -e "s/>=/==/g" -e "/psycopg2/d" -e "s/pyopenssl==16.0.0/pyopenssl==17.0.0/" pyproject.toml'
 
     # Install Synapse itself. This won't update any libraries.
     pip install ".[all, test]"