diff --git a/include/polarssl/dhm.h b/include/polarssl/dhm.h
index 48d9268..0a32bd9 100644
--- a/include/polarssl/dhm.h
+++ b/include/polarssl/dhm.h
@@ -218,7 +218,8 @@
  *
  * \param ctx      DHM context
  * \param output   destination buffer
- * \param olen     number of chars written
+ * \param olen     on entry, must hold the size of the destination buffer
+ *                 on exit, holds the actual number of bytes written
  *
  * \return         0 if successful, or an POLARSSL_ERR_DHM_XXX error code
  */