diff --git a/include/polarssl/arc4.h b/include/polarssl/arc4.h
index aa8feaa..9333265 100644
--- a/include/polarssl/arc4.h
+++ b/include/polarssl/arc4.h
@@ -55,7 +55,7 @@
  *
  * \param ctx      ARC4 context to be initialized
  * \param key      the secret key
- * \param keylen   length of the key
+ * \param keylen   length of the key, in bytes
  */
 void arc4_setup( arc4_context *ctx, const unsigned char *key, unsigned int keylen );