diff --git a/include/mbedtls/bignum.h b/include/mbedtls/bignum.h
index d03b983..3c9eeea 100644
--- a/include/mbedtls/bignum.h
+++ b/include/mbedtls/bignum.h
@@ -299,7 +299,7 @@
  *
  * \param X        MPI to use
  */
-size_t mbedtls_mpi_msb( const mbedtls_mpi *X );
+size_t mbedtls_mpi_bitlen( const mbedtls_mpi *X );
 
 /**
  * \brief          Return the total size in bytes