diff --git a/src/ast/for_each_expr.cpp b/src/ast/for_each_expr.cpp
index 6b7ebd1ee..2501222b8 100644
--- a/src/ast/for_each_expr.cpp
+++ b/src/ast/for_each_expr.cpp
@@ -65,7 +65,7 @@ bool has_skolem_functions(expr * n) {
 }
 
 subterms::subterms(expr_ref_vector const& es): m_es(es) {}
-subterms::subterms(expr_ref& e) : m_es(e.m()) { m_es.push_back(e); }
+subterms::subterms(const expr_ref& e) : m_es(e.m()) { m_es.push_back(e); }
 subterms::iterator subterms::begin() { return iterator(*this, true); }
 subterms::iterator subterms::end() { return iterator(*this, false); }
 subterms::iterator::iterator(subterms& f, bool start): m_es(f.m_es) {
diff --git a/src/ast/for_each_expr.h b/src/ast/for_each_expr.h
index 9da46d947..29d11a6e7 100644
--- a/src/ast/for_each_expr.h
+++ b/src/ast/for_each_expr.h
@@ -182,7 +182,7 @@ public:
         bool operator!=(iterator const& other) const;
     };
     subterms(expr_ref_vector const& es);
-    subterms(expr_ref& e);
+    subterms(const expr_ref& e);
     iterator begin();
     iterator end();
 };