diff --git a/library/bignum.c b/library/bignum.c
index 218e50b..9f11a70 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -762,6 +762,11 @@
     if( X != A )
         MPI_CHK( mpi_copy( X, A ) );
 
+    /*
+     * X should always be positive as a result of unsigned substractions.
+     */
+    X->s = 1;
+
     ret = 0;
 
     for( n = B->n - 1; n >= 0; n-- )