diff --git a/src/parser/FStar.Parser.AST.fs b/src/parser/FStar.Parser.AST.fs
index bab4647..6134448 100644
--- a/src/parser/FStar.Parser.AST.fs
+++ b/src/parser/FStar.Parser.AST.fs
@@ -492,7 +492,7 @@ let compile_op arity s r =
       |':' -> "Colon"
       |'$' -> "Dollar"
       |'.' -> "Dot"
-      | c -> raise_error (Fatal_UnexpectedOperatorSymbol, "Unexpected operator symbol: '" ^ string_of_char c ^ "'") r
+      | c -> "u" ^ (Util.string_of_int (Util.int_of_char c))
     in
     match s with
     | ".[]<-" -> "op_String_Assignment"