diff --git a/ch04.asciidoc b/ch04.asciidoc
index 0482cc33..abff2392 100644
--- a/ch04.asciidoc
+++ b/ch04.asciidoc
@@ -129,7 +129,9 @@ The bitcoin private key is just a number. You can pick your private keys
 randomly using just a coin, pencil, and paper: toss a coin 256 times and
 you have the binary digits of a random private key you can use in a
 bitcoin wallet. The public key can then be generated from the private
-key.
+key.  Be careful, though, as any process that's less than completely
+random can significantly reduce the security of your private key and the
+bitcoins it controls.
 ====
 
 ===== Generating a private key from a random number