diff --git a/widget/fs.lua b/widget/fs.lua
index 48d80b7..b3a2dad 100644
--- a/widget/fs.lua
+++ b/widget/fs.lua
@@ -137,7 +137,7 @@ local function factory(args)
     function fs.update(callback)
         Gio.Async.start(gears.protected_call.call)(function()
             update_synced()
-            if callback then
+            if type(callback) == "function" and callback then
                 callback()
             end
         end)