diff --git a/src/thumbnailer_shared.lua b/src/thumbnailer_shared.lua
index 509bda1..b0e5dfd 100644
--- a/src/thumbnailer_shared.lua
+++ b/src/thumbnailer_shared.lua
@@ -280,7 +280,7 @@ function Thumbnailer:register_client()
         local duration = mp.get_property_native("duration")
         local max_duration = thumbnailer_options.autogenerate_max_duration
 
-        if self.state.available and thumbnailer_options.autogenerate then
+        if duration ~= nil and self.state.available and thumbnailer_options.autogenerate then
             -- Notify if autogenerate is on and video is not too long
             if duration < max_duration or max_duration == 0 then
                 self:start_worker_jobs()