diff --git a/cron/cron.php b/cron/cron.php
index e1942c5c..604d4dba 100644
--- a/cron/cron.php
+++ b/cron/cron.php
@@ -9,10 +9,15 @@ if (php_sapi_name() !== 'cli') {
 }
 
 // Only one run at a time. Autopay charges cards, so an overlapping run (the previous
-// run still going when the next one fires) could bill the same invoice twice. The
-// handle is held for the life of the process and released when it exits.
-$cron_lock_handle = fopen(sys_get_temp_dir() . '/itflow_cron.lock', 'c');
-if ($cron_lock_handle === false || !flock($cron_lock_handle, LOCK_EX | LOCK_NB)) {
+// run still going when the next one fires) could bill the same invoice twice. The lock
+// file is named per install so separate ITFlow instances on one host don't block each
+// other, and the handle is held for the life of the process, released when it exits.
+$cron_lock_file = sys_get_temp_dir() . '/itflow_cron_' . md5(__DIR__) . '.lock';
+$cron_lock_handle = fopen($cron_lock_file, 'c');
+if ($cron_lock_handle === false) {
+    die("Cannot open the cron lock file at $cron_lock_file - check permissions and open_basedir.\n");
+}
+if (!flock($cron_lock_handle, LOCK_EX | LOCK_NB)) {
     die("Cron is already running - exiting.\n");
 }