diff --git a/lib/Electrum.js b/lib/Electrum.js
index 58822a2..e108690 100644
--- a/lib/Electrum.js
+++ b/lib/Electrum.js
@@ -7,6 +7,9 @@ var Key = require('./Key'),
 /**
  * Pre-BIP32 Electrum public key derivation (electrum <2.0)
  *
+ * For now, this class can only understands master public keys.
+ * It doesn't support derivation from a private master key (TODO).
+ *
  * @example examples/ElectrumMPK.js
  */
 function Electrum (master_public_key) {