diff --git a/jenkins/prepare_synapse.sh b/jenkins/prepare_synapse.sh
index ffcb1cfab..219828fa7 100755
--- a/jenkins/prepare_synapse.sh
+++ b/jenkins/prepare_synapse.sh
@@ -1,5 +1,7 @@
 #! /bin/bash
 
+set -eux
+
 cd "`dirname $0`/.."
 
 TOX_DIR=$WORKSPACE/.tox
@@ -14,7 +16,16 @@ fi
 tox -e py27 --notest -v
 
 TOX_BIN=$TOX_DIR/py27/bin
-$TOX_BIN/pip install setuptools
+
+# cryptography 2.2 requires setuptools >= 18.5.
+#
+# older versions of virtualenv (?) give us a virtualenv with the same version
+# of setuptools as is installed on the system python (and tox runs virtualenv
+# under python3, so we get the version of setuptools that is installed on that).
+#
+# anyway, make sure that we have a recent enough setuptools.
+$TOX_BIN/pip install 'setuptools>=18.5'
+
 { python synapse/python_dependencies.py
   echo lxml psycopg2
 } | xargs $TOX_BIN/pip install