diff --git a/common/content/completion.js b/common/content/completion.js
index b59304c2..192dfef9 100644
--- a/common/content/completion.js
+++ b/common/content/completion.js
@@ -285,7 +285,7 @@ CompletionContext.prototype = {
     set completions(items)
     {
         // Accept a generator
-        if (!(items instanceof Array))
+        if (!("length" in items))
             items = [x for (x in Iterator(items))];
         delete this.cache.filtered;
         delete this.cache.filter;