diff --git a/src/ast/rewriter/seq_rewriter.cpp b/src/ast/rewriter/seq_rewriter.cpp
index 186abda48..e5cac969f 100644
--- a/src/ast/rewriter/seq_rewriter.cpp
+++ b/src/ast/rewriter/seq_rewriter.cpp
@@ -367,6 +367,9 @@ br_status seq_rewriter::mk_app_core(func_decl * f, unsigned num_args, expr * con
         SASSERT(num_args == 2);
         return mk_re_concat(args[0], args[1], result);
     case OP_RE_UNION:
+        if (num_args == 1) {
+            result = args[0]; return BR_DONE;
+        }
         SASSERT(num_args == 2);
         return mk_re_union(args[0], args[1], result);
     case OP_RE_RANGE: