diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index 84f47e1..a2bb339 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -150,7 +150,9 @@
 mbedtls_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.
  */