diff --git a/extensions/PhabBugz/lib/Revision.pm b/extensions/PhabBugz/lib/Revision.pm
index 43dd93dd69..23f1a7984f 100644
--- a/extensions/PhabBugz/lib/Revision.pm
+++ b/extensions/PhabBugz/lib/Revision.pm
@@ -46,8 +46,10 @@ has view_policy      => (is => 'ro',   isa => Str);
 has edit_policy      => (is => 'ro',   isa => Str);
 has stack_graph_raw  => (
     is => 'ro',
-    isa => Dict [
-        slurpy Any
+    isa => Maybe [
+      Dict [
+          slurpy Any
+      ]
     ],
 );
 has subscriber_count => (is => 'ro',   isa => Int);