diff --git a/include/polarssl/entropy.h b/include/polarssl/entropy.h
index 3b5d692..f5fa928 100644
--- a/include/polarssl/entropy.h
+++ b/include/polarssl/entropy.h
@@ -183,7 +183,7 @@
  *
  * \param data      Entropy context
  * \param output    Buffer to fill
- * \param len       Length of buffer
+ * \param len       Number of bytes desired, must be at most ENTROPY_BLOCK_SIZE
  *
  * \return          0 if successful, or POLARSSL_ERR_ENTROPY_SOURCE_FAILED
  */