From 0f8d2d1d51b814edda853dacd8a7b88a45fad33a Mon Sep 17 00:00:00 2001
From: Nikolaj Bjorner <nbjorner@microsoft.com>
Date: Sat, 10 Jul 2021 14:47:51 +0200
Subject: [PATCH] fix #5399

---
 src/ast/rewriter/seq_rewriter.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index 9049ad971..6e0d0f7b7 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -1775,6 +1775,10 @@ br_status seq_rewriter::mk_seq_replace(expr* a, expr* b, expr* c, expr_ref& resu
         result = str().mk_concat(c, a);
         return BR_REWRITE1;
     }
+    if (str().is_empty(a) && str().is_empty(c)) {
+        result = a;
+        return BR_DONE;
+    }
 
     m_lhs.reset();
     str().get_concat(a, m_lhs);