--- a/taskcluster/scripts/run-task
+++ b/taskcluster/scripts/run-task
@@ -528,16 +528,20 @@ def main(args):
             return EXIT_PURGE_CACHE
 
     if 'TASKCLUSTER_VOLUMES' in os.environ:
         volumes = os.environ['TASKCLUSTER_VOLUMES'].split(';')
         del os.environ['TASKCLUSTER_VOLUMES']
     else:
         volumes = []
 
+    if volumes and not IS_POSIX:
+        print('assertion failed: volumes not expected on Windows')
+        return 1
+
     # Sanitize volumes.
     for volume in volumes:
         # If a volume is a cache, it was dealt with above.
         if volume in caches:
             print_line(b'volume', b'volume %s is a cache\n' % volume)
             continue
 
         configure_volume_posix(volume, user, group, running_as_root)