diff --git a/readme.md b/readme.md
index cbbf43f..f8a00ef 100644
--- a/readme.md
+++ b/readme.md
@@ -91,7 +91,9 @@ Adds `scalar` to the given key pair where scalar is a 32 byte buffer (possibly
 generated with `ed25519_create_seed`), generating a new key pair. You can
 calculate the public key sum without knowing the private key and vice versa by
 passing in NULL for the key you don't know. This is useful for enforcing
-randomness on a key pair while only knowing the public key, among other things. 
+randomness on a key pair while only knowing the public key, among other things.
+Warning: the last bit of the scalar is ignored - if comparing scalars make sure
+to clear it with `scalar[31] &= 127`.
 
 Example
 -------