diff --git a/library/ecp.c b/library/ecp.c
index 8b32caa..8943837 100644
--- a/library/ecp.c
+++ b/library/ecp.c
@@ -2526,7 +2526,7 @@
     MBEDTLS_MPI_CHK( ecp_randomize_mxz( grp, &RP, f_rng, p_rng ) );
 
     /* Loop invariant: R = result so far, RP = R + P */
-    i = mbedtls_mpi_bitlen( m ); /* one past the (zero-based) most significant bit */
+    i = grp->nbits + 1; /* one past the (zero-based) required msb for private keys */
     while( i-- > 0 )
     {
         b = mbedtls_mpi_get_bit( m, i );