diff --git a/src/ast/expr_substitution.cpp b/src/ast/expr_substitution.cpp
index 3ce038250..f9333c443 100644
--- a/src/ast/expr_substitution.cpp
+++ b/src/ast/expr_substitution.cpp
@@ -16,8 +16,9 @@ Author:
 Notes:
 
 --*/
-#include "ast/expr_substitution.h"
 #include "util/ref_util.h"
+#include "ast/expr_substitution.h"
+#include "ast/ast_pp.h"
 
 typedef obj_map<expr, proof*> expr2proof;
 typedef obj_map<expr, expr_dependency*> expr2expr_dependency;
@@ -58,7 +59,7 @@ expr_substitution::~expr_substitution() {
 
 std::ostream& expr_substitution::display(std::ostream& out) {
     for (auto & kv : m_subst) {
-        out << expr_ref(kv.m_key, m()) << " |-> " << expr_ref(kv.m_value, m()) << "\n";
+        out << mk_pp(kv.m_key, m()) << " |-> " << mk_pp(kv.m_value, m()) << "\n";
     }
     return out;
 }