diff --git a/library/pk.c b/library/pk.c
index d6ea912..0ba334b 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -728,7 +728,7 @@
 
         /* export the private key material in the format PSA wants */
         ec = mbedtls_pk_ec( *pk );
-        d_len = ( ec->grp.nbits + 7 ) / 8;
+        d_len = PSA_BITS_TO_BYTES( ec->grp.nbits );
         if( ( ret = mbedtls_mpi_write_binary( &ec->d, d, d_len ) ) != 0 )
             return( ret );