diff --git a/private/self-signed-cert.rkt b/private/self-signed-cert.rkt
index c1f0f07..81ed80c 100644
--- a/private/self-signed-cert.rkt
+++ b/private/self-signed-cert.rkt
@@ -27,11 +27,11 @@
   (private-key certificate))
 
 (c:define/contract (private-key ssc)
-  (c:-> self-signed-cert? bytes?)
+  (c:-> self-signed-cert? string?)
   (self-signed-cert-private-key ssc))
 
 (c:define/contract (certificate ssc)
-  (c:-> self-signed-cert? bytes?)
+  (c:-> self-signed-cert? string?)
   (self-signed-cert-certificate ssc))
 
 (define x509-cert certificate)