diff --git a/src/ast/ast.h b/src/ast/ast.h
index f71d5135c..75d3eb0f2 100644
--- a/src/ast/ast.h
+++ b/src/ast/ast.h
@@ -2021,8 +2021,8 @@ public:
     app * mk_not(expr * n) { return mk_app(m_basic_family_id, OP_NOT, n); }
     app * mk_distinct(unsigned num_args, expr * const * args);
     app * mk_distinct_expanded(unsigned num_args, expr * const * args);
-    app * mk_true() { return m_true; }
-    app * mk_false() { return m_false; }
+    app * mk_true() const { return m_true; }
+    app * mk_false() const { return m_false; }
     app * mk_bool_val(bool b) { return b?m_true:m_false; }
     app * mk_interp(expr * arg) { return mk_app(m_basic_family_id, OP_INTERP, arg); }