diff --git a/openssl/src/pkey.rs b/openssl/src/pkey.rs
index 7f03124416d95dda055937b2bab052486a537406..6973154f38cb9507799d34acd0caca58a633ff9c 100644
--- a/openssl/src/pkey.rs
+++ b/openssl/src/pkey.rs
@@ -117,6 +117,7 @@ impl PKey {
     }
 
     /// Creates a new `PKey` containing an HMAC key.
+    /// Note: To compute HMAC values, use the `sign` module.
     pub fn hmac(key: &[u8]) -> Result<PKey, ErrorStack> {
         unsafe {
             assert!(key.len() <= c_int::max_value() as usize);