diff --git a/font-face.rkt b/font-face.rkt
index b650618..3e97c3b 100644
--- a/font-face.rkt
+++ b/font-face.rkt
@@ -1,6 +1,6 @@
 #lang racket/base
 (require "core.rkt")
-(require net/url-structs net/base64 racket/file racket/format racket/list)
+(require net/url-structs net/base64 racket/file racket/format racket/list sugar/unstable/string)
 (provide (all-defined-out))
 
 (module+ test (require rackunit))
diff --git a/math.rkt b/math.rkt
index 727bfbc..2c38eb3 100644
--- a/math.rkt
+++ b/math.rkt
@@ -1,5 +1,5 @@
 #lang racket/base
-(require "core.rkt" racket/match racket/contract racket/string racket/list sugar/container sugar/coerce)
+(require "core.rkt" racket/match racket/contract racket/string racket/list sugar/unstable/container sugar/coerce)
 
 (define/contract (css-unit? x)
   (any/c . -> . boolean?)