diff --git a/src/effects.js b/src/effects.js
index d1d873def..f3163ebbf 100644
--- a/src/effects.js
+++ b/src/effects.js
@@ -256,7 +256,7 @@ jQuery.extend({
 		};
 
 		opt.duration = jQuery.fx.off ? 0 : typeof opt.duration === "number" ? opt.duration :
-			jQuery.fx.speeds[opt.duration] || jQuery.fx.speeds._default;
+			opt.duration in jQuery.fx.speeds ? jQuery.fx.speeds[opt.duration] : jQuery.fx.speeds._default;
 
 		// Queueing
 		opt.old = opt.complete;