diff --git a/programs/pkey/rsa_sign.c b/programs/pkey/rsa_sign.c
index e2d39fc..895880f 100644
--- a/programs/pkey/rsa_sign.c
+++ b/programs/pkey/rsa_sign.c
@@ -100,7 +100,7 @@
         goto exit;
     }
 
-    rsa.len = ( mbedtls_mpi_msb( &rsa.N ) + 7 ) >> 3;
+    rsa.len = ( mbedtls_mpi_bitlen( &rsa.N ) + 7 ) >> 3;
 
     fclose( f );