diff --git a/Ooui/Style.cs b/Ooui/Style.cs
index 3460e22..1d41705 100644
--- a/Ooui/Style.cs
+++ b/Ooui/Style.cs
@@ -432,6 +432,8 @@ namespace Ooui
         }
         static string AddUrl(object val)
         {
+            if (val == null)
+                return null;
             return String.Format("url('{0}')", val);
         }