diff --git a/unstable/lens/substring.rkt b/unstable/lens/substring.rkt
index 2eccc20..0d167f3 100644
--- a/unstable/lens/substring.rkt
+++ b/unstable/lens/substring.rkt
@@ -1,6 +1,6 @@
 #lang racket/base
 
-(require racket/contract/base)
+(require racket/contract/base unstable/contract)
 
 (provide
  (contract-out