diff --git a/b2/bom.c b/b2/bom.c
index 89cb31a..0da4a10 100644
--- a/b2/bom.c
+++ b/b2/bom.c
@@ -146,11 +146,13 @@ int bom_subst(struct bom *b, const struct subst *sub)
 
 	/* run substitutions */
 	res = substitute(sub, vars, &out);
-	free_vars(vars);
-	if (res)
-		b->vars = out;
-	else
+	if (res) {
+		b->vars = merge_vars(vars, out);
+		free_vars(out);
+	} else {
 		n_bom--;
+	}
+	free_vars(vars);
 	return res;
 }