--- a/src/pyams_default_theme/features/search/portlet/templates/search-panels.pt	Mon Jun 22 18:31:49 2020 +0200
+++ b/src/pyams_default_theme/features/search/portlet/templates/search-panels.pt	Fri Jun 26 12:58:51 2020 +0200
@@ -8,7 +8,14 @@
 	<form action="${tales:absolute_url(context)}"
 		  id="search-results">
 		<input type="hidden" name="user_search" value="${request.params.get('user_search')}" />
-		<input type="hidden" name="tag" value="${request.params.get('tag')}" />
+		<tal:var define="tag request.params.get('tag')"
+				 condition="tag">
+			<input type="hidden" name="tag" value="${tag}" />
+		</tal:var>
+		<tal:var define="collection request.params.get('collection')"
+				 condition="collection">
+			<input type="hidden" name="collection" value="${collection}" />
+		</tal:var>
 		<input type="hidden" name="order_by" value="${request.params.get('order_by') or context.order_by}" />
 		<input type="hidden" name="start" value="${start}" />
 		<input type="hidden" name="length" value="${length}" />