diff --git a/library/server/wsf/router/filter/wsf_filter.e b/library/server/wsf/router/filter/wsf_filter.e
index 66922784..bf426418 100644
--- a/library/server/wsf/router/filter/wsf_filter.e
+++ b/library/server/wsf/router/filter/wsf_filter.e
@@ -29,6 +29,8 @@ feature -- Element change
 
 	set_next (a_next: like next)
 			-- Set `next' to `a_next'
+		require
+			a_next_is_not_current: a_next /= Current
 		do
 			next := a_next
 		ensure