diff --git a/library/bignum.c b/library/bignum.c
index 5c1de68..218e50b 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -683,6 +683,11 @@
 
     if( X != A )
         MPI_CHK( mpi_copy( X, A ) );
+   
+    /*
+     * X should always be positive as a result of unsigned additions.
+     */
+    X->s = 1;
 
     for( j = B->n - 1; j >= 0; j-- )
         if( B->p[j] != 0 )