diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index df25bd1..8e1687b 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -188,7 +188,9 @@
 mpi;
 
 /**
- * \brief           Initialize one MPI
+ * \brief           Initialize one MPI (make internal references valid)
+ *                  This just makes it ready to be set or freed,
+ *                  but does not define a value for the MPI.
  *
  * \param X         One MPI to initialize.
  */