diff --git a/src/applications/herald/typeahead/HeraldRuleDatasource.php b/src/applications/herald/typeahead/HeraldRuleDatasource.php
--- a/src/applications/herald/typeahead/HeraldRuleDatasource.php
+++ b/src/applications/herald/typeahead/HeraldRuleDatasource.php
@@ -22,7 +22,8 @@
     $query = id(new HeraldRuleQuery())
       ->setViewer($viewer);
 
-    if (preg_match('/^[hH]\d+\z/', $raw_query)) {
+    if (phutil_nonempty_string($raw_query) &&
+        preg_match('/^[hH]\d+\z/', $raw_query)) {
       $id = trim($raw_query, 'hH');
       $id = (int)$id;
       $query->withIDs(array($id));