diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index c343812fe..80fee36d4 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -335,6 +335,9 @@ br_status seq_rewriter::mk_app_core(func_decl * f, unsigned num_args, expr * con
         SASSERT(num_args == 1);
         return mk_re_opt(args[0], result);
     case OP_RE_CONCAT:
+        if (num_args == 1) {
+            result = args[0]; return BR_DONE;
+        }
         SASSERT(num_args == 2);
         return mk_re_concat(args[0], args[1], result);
     case OP_RE_UNION:
diff --git a/src/ast/seq_decl_plugin.cpp b/src/ast/seq_decl_plugin.cpp
index caf8bd5cb..e9d87b90b 100644
--- a/src/ast/seq_decl_plugin.cpp
+++ b/src/ast/seq_decl_plugin.cpp
@@ -555,7 +555,7 @@ func_decl* seq_decl_plugin::mk_assoc_fun(decl_kind k, unsigned arity, sort* cons
     }
     match_right_assoc(*m_sigs[k], arity, domain, range, rng);
     func_decl_info info(m_family_id, k_seq);
-    info.set_right_associative();
+    info.set_right_associative(true);
     return m.mk_func_decl(m_sigs[(rng == m_string)?k_string:k_seq]->m_name, rng, rng, rng, info);
 }
 
diff --git a/src/ast/well_sorted.cpp b/src/ast/well_sorted.cpp
index cb9ee3f61..b0afe566b 100644
--- a/src/ast/well_sorted.cpp
+++ b/src/ast/well_sorted.cpp
@@ -44,7 +44,8 @@ struct well_sorted_proc {
     void operator()(app * n) {   
         unsigned num_args  = n->get_num_args();
         func_decl * decl   = n->get_decl();
-        if (num_args != decl->get_arity() && !decl->is_associative()) {
+        if (num_args != decl->get_arity() && !decl->is_associative() && 
+            !decl->is_right_associative() && !decl->is_left_associative()) {
             TRACE("ws", tout << "unexpected number of arguments.\n" << mk_ismt2_pp(n, m_manager););
             warning_msg("unexpected number of arguments.");
             m_error = true;