diff --git a/bench/insert-string-set b/bench/insert-string-set
new file mode 100755
index 0000000..083f756
Binary files /dev/null and b/bench/insert-string-set differ
diff --git a/src/string-set.sml b/src/string-set.sml
index 64c1ac3..8a6f562 100644
--- a/src/string-set.sml
+++ b/src/string-set.sml
@@ -67,7 +67,10 @@ struct
   val empty =
     CHILDREN {keys = Vector.fromList [], children = Vector.fromList []}
 
-  fun isEmpty trie = trie = empty
+  fun isEmpty trie =
+    case trie of
+      CHILDREN {keys, ...} => Vector.length keys = 0
+    | _ => false
 
   fun fromString str =
     if String.size str > 0 then