diff --git a/include/polarssl/bignum.h b/include/polarssl/bignum.h
index 5631af3..635c5d7 100644
--- a/include/polarssl/bignum.h
+++ b/include/polarssl/bignum.h
@@ -136,6 +136,7 @@
           defined(__s390x__) || defined(__mips64) ) )
        typedef  int64_t t_sint;
        typedef uint64_t t_uint;
+       /* mbedtls_t_udbl defined as 128-bit unsigned int */
        typedef unsigned int t_udbl __attribute__((mode(TI)));
        #define POLARSSL_HAVE_UDBL
     #else