diff --git a/src/ast/rewriter/var_subst.cpp b/src/ast/rewriter/var_subst.cpp
index 4d4c17865..711ca2b51 100644
--- a/src/ast/rewriter/var_subst.cpp
+++ b/src/ast/rewriter/var_subst.cpp
@@ -59,7 +59,10 @@ expr_ref var_subst::operator()(expr * n, unsigned num_args, expr * const * args)
                 new_args.push_back(arg);
             else {
                 unsigned idx = to_var(arg)->get_idx();
-                new_args.push_back(m_std_order ? args[num_args - idx - 1] : args[idx]);
+                expr* new_arg = m_std_order ? args[num_args - idx - 1] : args[idx];
+                if (!new_arg)
+                    new_arg = arg;
+                new_args.push_back(new_arg);
             }
         }
         result = m.mk_app(to_app(n)->get_decl(), new_args.size(), new_args.data());