diff --git a/image_filter.t b/image_filter.t
index a106ce8..b9438cd 100644
--- a/image_filter.t
+++ b/image_filter.t
@@ -259,7 +259,7 @@
 ###############################################################################
 
 sub gif_size {
-	join ' ', unpack("x6S2", http_get_body(@_));
+	join ' ', unpack("x6v2", http_get_body(@_));
 }
 
 sub http_get_body {