diff --git a/scripts/setup/upgrade-postgresql b/scripts/setup/upgrade-postgresql
index 72653b8e7d..9f9cda2978 100755
--- a/scripts/setup/upgrade-postgresql
+++ b/scripts/setup/upgrade-postgresql
@@ -6,6 +6,12 @@ if [ "$EUID" -ne 0 ]; then
     exit 1
 fi
 
+# Force a known locale; some OS X terminals set an invalid LC_CTYPE
+# which crashes pg_upgradecluster
+export LC_ALL=C.UTF-8
+export LANG=C.UTF-8
+export LANGUAGE=C.UTF-8
+
 UPGRADE_TO=${1:-14}
 UPGRADE_FROM=$(crudini --get /etc/zulip/zulip.conf postgresql version)
 ZULIP_PATH="$(dirname "$0")/../.."