diff --git a/src/ast/rewriter/seq_axioms.cpp b/src/ast/rewriter/seq_axioms.cpp
index 3ef433138..2d48cfe4e 100644
--- a/src/ast/rewriter/seq_axioms.cpp
+++ b/src/ast/rewriter/seq_axioms.cpp
@@ -792,8 +792,6 @@ namespace seq {
         es.reverse();
         eq = m.mk_eq(seq.str.mk_ubv2s(b), seq.str.mk_concat(es, seq.str.mk_string_sort()));
         add_clause(~ge10k, ge10k1, eq);
-        ubv2ch_axiom(bv_sort);
-
     }
 
     void axioms::ubv2ch_axiom(sort* bv_sort) {
diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index 8745326b0..0bd653232 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -2209,6 +2209,12 @@ br_status seq_rewriter::mk_str_is_digit(expr* a, expr_ref& result) {
 
 
 br_status seq_rewriter::mk_str_ubv2s(expr* a, expr_ref& result) {
+    bv_util bv(m());
+    rational val;
+    if (bv.is_numeral(a, val)) {
+        result = str().mk_string(zstring(val));
+        return BR_DONE;
+    }
     return BR_FAILED;
 }
 
diff --git a/src/smt/seq_axioms.h b/src/smt/seq_axioms.h
index cd7f38064..16091fcf4 100644
--- a/src/smt/seq_axioms.h
+++ b/src/smt/seq_axioms.h
@@ -80,6 +80,7 @@ namespace smt {
         void add_stoi_axiom(expr* e, unsigned k) { m_ax.stoi_axiom(e, k); }
         void add_itos_axiom(expr* s, unsigned k) { m_ax.itos_axiom(s, k); }
         void add_ubv2s_axiom(expr* b, unsigned k) { m_ax.ubv2s_axiom(b, k); }
+        void add_ubv2ch_axioms(sort* s) { m_ax.ubv2ch_axiom(s); }
         void add_lt_axiom(expr* n) { m_ax.lt_axiom(n); }
         void add_le_axiom(expr* n) { m_ax.le_axiom(n); }
         void add_is_digit_axiom(expr* n) { m_ax.is_digit_axiom(n); }
diff --git a/src/smt/theory_seq.cpp b/src/smt/theory_seq.cpp
index 7e4eda114..599646b5b 100644
--- a/src/smt/theory_seq.cpp
+++ b/src/smt/theory_seq.cpp
@@ -1579,6 +1579,14 @@ bool theory_seq::check_ubv_string(expr* e) {
         k++;
         value = div(value, rational(10));
     }
+    bool has_sort = false;
+    for (auto* e2 : m_has_ubv_axiom) {
+        expr* b2 = nullptr;
+        VERIFY(m_util.str.is_ubv2s(e2, b2));
+        has_sort |= b2->get_sort() == b->get_sort();
+    }
+    if (!has_sort)
+        m_ax.add_ubv2ch_axioms(b->get_sort());
     m_has_ubv_axiom.insert(e);
     m_trail_stack.push(insert_obj_trail<expr>(m_has_ubv_axiom, e));
     m_ax.add_ubv2s_axiom(b, k);