diff --git a/imgkit/imgkit.py b/imgkit/imgkit.py
index 9cc1bfc..1d16da9 100644
--- a/imgkit/imgkit.py
+++ b/imgkit/imgkit.py
@@ -82,6 +82,9 @@ def _command(self, path=None):
         if '--xvfb' in options:
             options.remove('--xvfb')
             yield self.xvfb
+            # auto servernum option to prevent failure on concurrent runs
+            # https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/348052
+            yield '-a'
 
         yield self.wkhtmltoimage