diff --git a/src/ast/ast.cpp b/src/ast/ast.cpp
index 2ded7c1b3a..ba0f9ff350 100644
--- a/src/ast/ast.cpp
+++ b/src/ast/ast.cpp
@@ -2210,6 +2210,9 @@ app * ast_manager::mk_app(func_decl * decl, unsigned num_args, expr * const * ar
     if (num_args == 1 && decl->is_chainable() && decl->get_arity() == 2) {
         r = mk_true();
     }
+    // riskier fix:
+//    else if (num_args == 1 && decl->is_left_associative() && is_app(args[0]))
+//        r = to_app(args[0]);
     else if (num_args > 2 && !decl->is_flat_associative()) {
         if (decl->is_right_associative()) {
             unsigned j = num_args - 1;
diff --git a/src/ast/seq_decl_plugin.cpp b/src/ast/seq_decl_plugin.cpp
index 75949316a0..6f71dec7e7 100644
--- a/src/ast/seq_decl_plugin.cpp
+++ b/src/ast/seq_decl_plugin.cpp
@@ -1695,14 +1695,20 @@ seq_util::rex::info seq_util::rex::mk_info_rec(app* e) const {
             //TBD: check if the character predicate contains uninterpreted symbols or is nonground or is unsat
             return info(true, l_false, 1, false);
         case OP_RE_CONCAT:
+            if (e->get_num_args() != 2)
+                return unknown_info;
             i1 = get_info_rec(e->get_arg(0));
             i2 = get_info_rec(e->get_arg(1));
             return i1.concat(i2, u.re.is_concat(e->get_arg(0)));
         case OP_RE_UNION:
+            if (e->get_num_args() != 2)
+                return unknown_info;
             i1 = get_info_rec(e->get_arg(0));
             i2 = get_info_rec(e->get_arg(1));
             return i1.disj(i2);
         case OP_RE_INTERSECT:
+            if (e->get_num_args() != 2)
+                return unknown_info;
             i1 = get_info_rec(e->get_arg(0));
             i2 = get_info_rec(e->get_arg(1));
             return i1.conj(i2);
@@ -1727,10 +1733,14 @@ seq_util::rex::info seq_util::rex::mk_info_rec(app* e) const {
                 upper_bound = e->get_decl()->get_parameter(1).get_int();
             return i1.loop(lower_bound, upper_bound);
         case OP_RE_DIFF:
+            if (e->get_num_args() != 2)
+                return unknown_info;
             i1 = get_info_rec(e->get_arg(0));
             i2 = get_info_rec(e->get_arg(1));
             return i1.diff(i2);
         case OP_RE_XOR:
+            if (e->get_num_args() != 2)
+                return unknown_info;
             i1 = get_info_rec(e->get_arg(0));
             i2 = get_info_rec(e->get_arg(1));
             return i1.xor_(i2);