diff --git a/zuul_sphinx/zuul.py b/zuul_sphinx/zuul.py
index bb669be..be0e97a 100644
--- a/zuul_sphinx/zuul.py
+++ b/zuul_sphinx/zuul.py
@@ -156,7 +156,7 @@ class ZuulDirective(Directive):
                     branches = job['branches']
                     if not isinstance(branches, list):
                         branches = [branches]
-                    variant = ', '.join(branches)
+                    variant = ', '.join([str(x) for x in branches])
                     lines.append('   :variant: %s' % variant)
                 lines.append('')
                 for l in job.get('description', '').split('\n'):